diff --git a/src/Module.zig b/src/Module.zig
index fdf61c4bf6..83252c76ad 100644
--- a/src/Module.zig
+++ b/src/Module.zig
@@ -1394,7 +1394,15 @@ pub const Fn = struct {
     /// there is a `TypedValue` here for each parameter of the function.
     /// Non-comptime parameters are marked with a `generic_poison` for the value.
     /// Non-anytype parameters are marked with a `generic_poison` for the type.
-    comptime_args: ?[*]TypedValue = null,
+    /// These never have .generic_poison for the Type
+    /// because the Type is needed to pass to `Type.eql` and for inserting comptime arguments
+    /// into the inst_map when analyzing the body of a generic function instantiation.
+    /// Instead, the is_anytype knowledge is communicated via `anytype_args`.
+    comptime_args: ?[*]TypedValue,
+    /// When comptime_args is null, this is undefined. Otherwise, this flags each
+    /// parameter and tells whether it is anytype.
+    /// TODO apply the same enhancement for param_names below to this field.
+    anytype_args: [*]bool,
     /// The ZIR instruction that is a function instruction. Use this to find
     /// the body. We store this rather than the body directly so that when ZIR
     /// is regenerated on update(), we can map this to the new corresponding
@@ -4782,18 +4790,24 @@ pub fn analyzeFnBody(mod: *Module, decl: *Decl, func: *Fn, arena: Allocator) Sem
 
             else => continue,
         };
-        if (func.comptime_args) |comptime_args| {
+
+        const param_ty = if (func.comptime_args) |comptime_args| t: {
             const arg_tv = comptime_args[total_param_index];
-            if (arg_tv.val.tag() != .generic_poison) {
-                // We have a comptime value for this parameter.
-                const arg = try sema.addConstant(arg_tv.ty, arg_tv.val);
-                sema.inst_map.putAssumeCapacityNoClobber(inst, arg);
-                total_param_index += 1;
-                continue;
-            }
-        }
-        const param_type = fn_ty_info.param_types[runtime_param_index];
-        const opt_opv = sema.typeHasOnePossibleValue(&inner_block, param.src, param_type) catch |err| switch (err) {
+
+            const arg_val = if (arg_tv.val.tag() != .generic_poison)
+                arg_tv.val
+            else if (arg_tv.ty.onePossibleValue()) |opv|
+                opv
+            else
+                break :t arg_tv.ty;
+
+            const arg = try sema.addConstant(arg_tv.ty, arg_val);
+            sema.inst_map.putAssumeCapacityNoClobber(inst, arg);
+            total_param_index += 1;
+            continue;
+        } else fn_ty_info.param_types[runtime_param_index];
+
+        const opt_opv = sema.typeHasOnePossibleValue(&inner_block, param.src, param_ty) catch |err| switch (err) {
             error.NeededSourceLocation => unreachable,
             error.GenericPoison => unreachable,
             error.ComptimeReturn => unreachable,
@@ -4801,7 +4815,7 @@ pub fn analyzeFnBody(mod: *Module, decl: *Decl, func: *Fn, arena: Allocator) Sem
             else => |e| return e,
         };
         if (opt_opv) |opv| {
-            const arg = try sema.addConstant(param_type, opv);
+            const arg = try sema.addConstant(param_ty, opv);
             sema.inst_map.putAssumeCapacityNoClobber(inst, arg);
             total_param_index += 1;
             runtime_param_index += 1;
@@ -4811,7 +4825,7 @@ pub fn analyzeFnBody(mod: *Module, decl: *Decl, func: *Fn, arena: Allocator) Sem
         inner_block.instructions.appendAssumeCapacity(arg_index);
         sema.air_instructions.appendAssumeCapacity(.{
             .tag = .arg,
-            .data = .{ .ty = param_type },
+            .data = .{ .ty = param_ty },
         });
         sema.inst_map.putAssumeCapacityNoClobber(inst, Air.indexToRef(arg_index));
         total_param_index += 1;
diff --git a/src/Sema.zig b/src/Sema.zig
index 8142132679..97fe3cb595 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -4707,6 +4707,8 @@ const GenericCallAdapter = struct {
     generic_fn: *Module.Fn,
     precomputed_hash: u64,
     func_ty_info: Type.Payload.Function.Data,
+    /// Unlike comptime_args, the Type here is not always present.
+    /// .generic_poison is used to communicate non-anytype parameters.
     comptime_tvs: []const TypedValue,
     target: std.Target,
 
@@ -4719,20 +4721,29 @@ const GenericCallAdapter = struct {
 
         const other_comptime_args = other_key.comptime_args.?;
         for (other_comptime_args[0..ctx.func_ty_info.param_types.len]) |other_arg, i| {
-            if (other_arg.ty.tag() != .generic_poison) {
-                // anytype parameter
-                if (!other_arg.ty.eql(ctx.comptime_tvs[i].ty, ctx.target)) {
+            const this_arg = ctx.comptime_tvs[i];
+            const this_is_comptime = this_arg.val.tag() != .generic_poison;
+            const other_is_comptime = other_arg.val.tag() != .generic_poison;
+            const this_is_anytype = this_arg.ty.tag() != .generic_poison;
+            const other_is_anytype = other_key.anytype_args[i];
+
+            if (other_is_anytype != this_is_anytype) return false;
+            if (other_is_comptime != this_is_comptime) return false;
+
+            if (this_is_anytype) {
+                // Both are anytype parameters.
+                if (!this_arg.ty.eql(other_arg.ty, ctx.target)) {
                     return false;
                 }
-            }
-            if (other_arg.val.tag() != .generic_poison) {
-                // comptime parameter
-                if (ctx.comptime_tvs[i].val.tag() == .generic_poison) {
-                    // No match because the instantiation has a comptime parameter
-                    // but the callsite does not.
-                    return false;
+                if (this_is_comptime) {
+                    // Both are comptime and anytype parameters with matching types.
+                    if (!this_arg.val.eql(other_arg.val, other_arg.ty, ctx.target)) {
+                        return false;
+                    }
                 }
-                if (!other_arg.val.eql(ctx.comptime_tvs[i].val, other_arg.ty, ctx.target)) {
+            } else if (this_is_comptime) {
+                // Both are comptime parameters but not anytype parameters.
+                if (!this_arg.val.eql(other_arg.val, other_arg.ty, ctx.target)) {
                     return false;
                 }
             }
@@ -5227,28 +5238,61 @@ fn instantiateGenericCall(
     const comptime_tvs = try sema.arena.alloc(TypedValue, func_ty_info.param_types.len);
     const target = sema.mod.getTarget();
 
-    for (func_ty_info.param_types) |param_ty, i| {
-        const is_comptime = func_ty_info.paramIsComptime(i);
-        if (is_comptime) {
-            const arg_src = call_src; // TODO better source location
-            const casted_arg = try sema.coerce(block, param_ty, uncasted_args[i], arg_src);
-            if (try sema.resolveMaybeUndefVal(block, arg_src, casted_arg)) |arg_val| {
-                if (param_ty.tag() != .generic_poison) {
-                    arg_val.hash(param_ty, &hasher, target);
+    {
+        var i: usize = 0;
+        for (fn_info.param_body) |inst| {
+            var is_comptime = false;
+            var is_anytype = false;
+            switch (zir_tags[inst]) {
+                .param => {
+                    is_comptime = func_ty_info.paramIsComptime(i);
+                },
+                .param_comptime => {
+                    is_comptime = true;
+                },
+                .param_anytype => {
+                    is_anytype = true;
+                    is_comptime = func_ty_info.paramIsComptime(i);
+                },
+                .param_anytype_comptime => {
+                    is_anytype = true;
+                    is_comptime = true;
+                },
+                else => continue,
+            }
+
+            if (is_comptime) {
+                const arg_src = call_src; // TODO better source location
+                const arg_ty = sema.typeOf(uncasted_args[i]);
+                const arg_val = try sema.resolveValue(block, arg_src, uncasted_args[i]);
+                arg_val.hash(arg_ty, &hasher, target);
+                if (is_anytype) {
+                    arg_ty.hashWithHasher(&hasher, target);
+                    comptime_tvs[i] = .{
+                        .ty = arg_ty,
+                        .val = arg_val,
+                    };
+                } else {
+                    comptime_tvs[i] = .{
+                        .ty = Type.initTag(.generic_poison),
+                        .val = arg_val,
+                    };
                 }
+            } else if (is_anytype) {
+                const arg_ty = sema.typeOf(uncasted_args[i]);
+                arg_ty.hashWithHasher(&hasher, target);
                 comptime_tvs[i] = .{
-                    // This will be different than `param_ty` in the case of `generic_poison`.
-                    .ty = sema.typeOf(casted_arg),
-                    .val = arg_val,
+                    .ty = arg_ty,
+                    .val = Value.initTag(.generic_poison),
                 };
             } else {
-                return sema.failWithNeededComptime(block, arg_src);
+                comptime_tvs[i] = .{
+                    .ty = Type.initTag(.generic_poison),
+                    .val = Value.initTag(.generic_poison),
+                };
             }
-        } else {
-            comptime_tvs[i] = .{
-                .ty = sema.typeOf(uncasted_args[i]),
-                .val = Value.initTag(.generic_poison),
-            };
+
+            i += 1;
         }
     }
 
@@ -5411,19 +5455,48 @@ fn instantiateGenericCall(
         errdefer new_func.deinit(gpa);
         assert(new_func == new_module_func);
 
+        const anytype_args = try new_decl_arena_allocator.alloc(bool, func_ty_info.param_types.len);
+        new_func.anytype_args = anytype_args.ptr;
         arg_i = 0;
         for (fn_info.param_body) |inst| {
+            var is_comptime = false;
+            var is_anytype = false;
             switch (zir_tags[inst]) {
-                .param_comptime, .param_anytype_comptime, .param, .param_anytype => {},
+                .param => {
+                    is_comptime = func_ty_info.paramIsComptime(arg_i);
+                },
+                .param_comptime => {
+                    is_comptime = true;
+                },
+                .param_anytype => {
+                    is_anytype = true;
+                    is_comptime = func_ty_info.paramIsComptime(arg_i);
+                },
+                .param_anytype_comptime => {
+                    is_anytype = true;
+                    is_comptime = true;
+                },
                 else => continue,
             }
+
+            // We populate the Type here regardless because it is needed by
+            // `GenericCallAdapter.eql` as well as function body analysis.
+            // Whether it is anytype is communicated by `anytype_args`.
             const arg = child_sema.inst_map.get(inst).?;
             const copied_arg_ty = try child_sema.typeOf(arg).copy(new_decl_arena_allocator);
-            if (child_sema.resolveMaybeUndefValAllowVariables(
-                &child_block,
-                .unneeded,
-                arg,
-            ) catch unreachable) |arg_val| {
+            anytype_args[arg_i] = is_anytype;
+
+            const arg_src = call_src; // TODO: better source location
+            if (try sema.typeRequiresComptime(block, arg_src, copied_arg_ty)) {
+                is_comptime = true;
+            }
+
+            if (is_comptime) {
+                const arg_val = (child_sema.resolveMaybeUndefValAllowVariables(
+                    &child_block,
+                    .unneeded,
+                    arg,
+                ) catch unreachable).?;
                 child_sema.comptime_args[arg_i] = .{
                     .ty = copied_arg_ty,
                     .val = try arg_val.copy(new_decl_arena_allocator),
@@ -5480,22 +5553,7 @@ fn instantiateGenericCall(
 
     const comptime_args = callee.comptime_args.?;
     const new_fn_info = callee.owner_decl.ty.fnInfo();
-    const runtime_args_len = count: {
-        var count: u32 = 0;
-        var arg_i: usize = 0;
-        for (fn_info.param_body) |inst| {
-            switch (zir_tags[inst]) {
-                .param_comptime, .param_anytype_comptime, .param, .param_anytype => {
-                    if (comptime_args[arg_i].val.tag() == .generic_poison) {
-                        count += 1;
-                    }
-                    arg_i += 1;
-                },
-                else => continue,
-            }
-        }
-        break :count count;
-    };
+    const runtime_args_len = @intCast(u32, new_fn_info.param_types.len);
     const runtime_args = try sema.arena.alloc(Air.Inst.Ref, runtime_args_len);
     {
         var runtime_i: u32 = 0;
@@ -5505,7 +5563,9 @@ fn instantiateGenericCall(
                 .param_comptime, .param_anytype_comptime, .param, .param_anytype => {},
                 else => continue,
             }
-            const is_runtime = comptime_args[total_i].val.tag() == .generic_poison;
+            const is_runtime = comptime_args[total_i].val.tag() == .generic_poison and
+                comptime_args[total_i].ty.hasRuntimeBits() and
+                !comptime_args[total_i].ty.comptimeOnly();
             if (is_runtime) {
                 const param_ty = new_fn_info.param_types[runtime_i];
                 const arg_src = call_src; // TODO: better source location
@@ -6562,6 +6622,7 @@ fn funcCommon(
         .zir_body_inst = func_inst,
         .owner_decl = sema.owner_decl,
         .comptime_args = comptime_args,
+        .anytype_args = undefined,
         .hash = hash,
         .lbrace_line = src_locs.lbrace_line,
         .rbrace_line = src_locs.rbrace_line,
diff --git a/src/value.zig b/src/value.zig
index beb8bc7620..713cacb7b0 100644
--- a/src/value.zig
+++ b/src/value.zig
@@ -954,6 +954,10 @@ pub const Value = extern union {
                 assert(ty.enumFieldCount() == 1);
                 break :blk 0;
             },
+            .enum_literal => i: {
+                const name = val.castTag(.enum_literal).?.data;
+                break :i ty.enumFieldIndex(name).?;
+            },
             // Assume it is already an integer and return it directly.
             else => return val,
         };
@@ -2023,6 +2027,11 @@ pub const Value = extern union {
     /// This function is used by hash maps and so treats floating-point NaNs as equal
     /// to each other, and not equal to other floating-point values.
     /// Similarly, it treats `undef` as a distinct value from all other values.
+    /// This function has to be able to support implicit coercion of `a` to `ty`. That is,
+    /// `ty` will be an exactly correct Type for `b` but it may be a post-coerced Type
+    /// for `a`. This function must act *as if* `a` has been coerced to `ty`. This complication
+    /// is required in order to make generic function instantiation effecient - specifically
+    /// the insertion into the monomorphized function table.
     pub fn eql(a: Value, b: Value, ty: Type, target: Target) bool {
         const a_tag = a.tag();
         const b_tag = b.tag();
@@ -2200,8 +2209,18 @@ pub const Value = extern union {
                 }
                 return order(a, b, target).compare(.eq);
             },
-            else => return order(a, b, target).compare(.eq),
+            .Optional => {
+                if (a.tag() != .opt_payload and b.tag() == .opt_payload) {
+                    var buffer: Payload.SubValue = .{
+                        .base = .{ .tag = .opt_payload },
+                        .data = a,
+                    };
+                    return eql(Value.initPayload(&buffer.base), b, ty, target);
+                }
+            },
+            else => {},
         }
+        return order(a, b, target).compare(.eq);
     }
 
     /// This function is used by hash maps and so treats floating-point NaNs as equal
diff --git a/test/behavior/generics.zig b/test/behavior/generics.zig
index 08f1e5bad9..2a18135fe0 100644
--- a/test/behavior/generics.zig
+++ b/test/behavior/generics.zig
@@ -306,3 +306,21 @@ test "anonymous struct return type referencing comptime parameter" {
     try expect(s.data == 1234);
     try expect(s.end == 5678);
 }
+
+test "generic function instantiation non-duplicates" {
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.os.tag == .wasi) return error.SkipZigTest;
+
+    const S = struct {
+        fn copy(comptime T: type, dest: []T, source: []const T) void {
+            @export(foo, .{ .name = "test_generic_instantiation_non_dupe" });
+            for (source) |s, i| dest[i] = s;
+        }
+
+        fn foo() callconv(.C) void {}
+    };
+    var buffer: [100]u8 = undefined;
+    S.copy(u8, &buffer, "hello");
+    S.copy(u8, &buffer, "hello2");
+}