Fix #2347.

common/changes/@typespec/http/openapi3-DuplicateRefs_2023-09-20-17-37.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@typespec/http",
+      "comment": "",
+      "type": "none"
+    }
+  ],
+  "packageName": "@typespec/http"
+}

common/changes/@typespec/openapi3/openapi3-DuplicateRefs_2023-09-20-17-37.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@typespec/openapi3",
+      "comment": "Fix issue where response schemas would sometimes appear as \"anyOf\" with duplicate identical references when they should just be a single schema reference.",
+      "type": "none"
+    }
+  ],
+  "packageName": "@typespec/openapi3"
+}

packages/http/src/responses.ts

@@ -140,7 +140,8 @@ function getResponseStatusCodes(
     }
   }
 
-  return codes;
+  // eliminate duplicate status codes
+  return [...new Set(codes)];
 }
 
 /**

packages/openapi3/test/openapi-output.test.ts

@@ -167,6 +167,28 @@ describe("openapi3: operations", () => {
 
     strictEqual(res.paths["/"].get.deprecated, true);
   });
+
+  it("define operation response structure", async () => {
+    const res = await openApiFor(
+      `      
+      model CustomUnauthorizedResponse {
+        @statusCode _: 401;
+        @body body: UnauthorizedResponse;
+      }
+      
+      op list(): CustomUnauthorizedResponse;
+      `
+    );
+    const responses = res.paths["/"].get.responses;
+    ok(responses);
+    ok(responses["401"]);
+    ok(responses["401"].content);
+    ok(responses["401"].content["application/json"]);
+    ok(responses["401"].content["application/json"].schema);
+    deepStrictEqual(responses["401"].content["application/json"].schema, {
+      $ref: "#/components/schemas/TypeSpec.Http.UnauthorizedResponse",
+    });
+  });
 });
 
 describe("openapi3: request", () => {