sema: add compile error for duplicate union field

src/Sema.zig

@@ -22215,7 +22215,22 @@ fn semaUnionFields(block: *Block, mod: *Module, union_obj: *Module.Union) Compil
         }
 
         const gop = union_obj.fields.getOrPutAssumeCapacity(field_name);
-        assert(!gop.found_existing);
+        if (gop.found_existing) {
+            const msg = msg: {
+                const tree = try sema.getAstTree(&block_scope);
+                const field_src = enumFieldSrcLoc(decl, tree.*, union_obj.node_offset, field_i);
+                const msg = try sema.errMsg(&block_scope, field_src, "duplicate union field: '{s}'", .{field_name});
+                errdefer msg.destroy(gpa);
+
+                const prev_field_index = union_obj.fields.getIndex(field_name).?;
+                const prev_field_src = enumFieldSrcLoc(decl, tree.*, union_obj.node_offset, prev_field_index);
+                try sema.mod.errNoteNonLazy(prev_field_src.toSrcLoc(decl), msg, "other field here", .{});
+                try sema.errNote(&block_scope, src, msg, "union declared here", .{});
+                break :msg msg;
+            };
+            return sema.failWithOwnedErrorMsg(&block_scope, msg);
+        }
+
         gop.value_ptr.* = .{
             .ty = try field_ty.copy(decl_arena_allocator),
             .abi_align = 0,

test/compile_errors/stage2/union_duplicate_field_definition.zig

@@ -0,0 +1,15 @@
+const U = union {
+    foo: u32,
+    foo: u32,
+};
+
+export fn entry() void {
+    const u: U = .{ .foo = 100 };
+    _ = u;
+}
+
+// duplicate union field name
+//
+// :3:5: error: duplicate union field: 'foo'
+// :2:5: note: other field here
+// :1:11: note: union declared here