Prevent duplicate HTTP headers when WriterInterceptor is used

Fixes: quarkusio#27325

independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/jaxrs/WriterInterceptorContextImpl.java

@@ -42,6 +42,8 @@ public WriterInterceptorContextImpl(ResteasyReactiveRequestContext context, Writ
     @Override
     public void proceed() throws IOException, WebApplicationException {
         Response response = context.getResponse().get();
+        // this is needed in order to avoid having the headers written out twice
+        context.serverResponse().setPreCommitListener(null);
         if (index == interceptors.length) {
             MessageBodyWriter effectiveWriter = writer;
             if (rediscoveryNeeded) {

independent-projects/resteasy-reactive/server/vertx/src/test/java/org/jboss/resteasy/reactive/server/vertx/test/response/CustomHeadersAndWriterInterceptorTest.java

@@ -0,0 +1,61 @@
+package org.jboss.resteasy.reactive.server.vertx.test.response;
+
+import static io.restassured.RestAssured.when;
+import static org.assertj.core.api.Assertions.*;
+import static org.hamcrest.Matchers.is;
+
+import io.restassured.http.Headers;
+import java.io.IOException;
+import java.util.Date;
+import javax.ws.rs.GET;
+import javax.ws.rs.Path;
+import javax.ws.rs.Produces;
+import javax.ws.rs.WebApplicationException;
+import javax.ws.rs.core.MediaType;
+import javax.ws.rs.core.Response;
+import javax.ws.rs.ext.Provider;
+import javax.ws.rs.ext.WriterInterceptor;
+import javax.ws.rs.ext.WriterInterceptorContext;
+import org.jboss.resteasy.reactive.server.vertx.test.framework.ResteasyReactiveUnitTest;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+public class CustomHeadersAndWriterInterceptorTest {
+
+    @RegisterExtension
+    static ResteasyReactiveUnitTest runner = new ResteasyReactiveUnitTest()
+            .withApplicationRoot((jar) -> jar
+                    .addClasses(TestResource.class, DummyWriterInterceptor.class));
+
+    @Test
+    void testResponseHeaders() {
+        Headers headers = when()
+                .get("/test")
+                .then()
+                .statusCode(200)
+                .header("etag", is("0"))
+                .extract().headers();
+        assertThat(headers.getList("etag")).hasSize(1);
+        assertThat(headers.getList("Last-Modified")).hasSize(1);
+    }
+
+    @Path("/test")
+    public static class TestResource {
+
+        @GET
+        @Produces(MediaType.TEXT_PLAIN)
+        public Response hello() {
+            return Response.ok("123").lastModified(new Date()).header("etag", 0).build();
+        }
+    }
+
+    @Provider
+    public static class DummyWriterInterceptor implements WriterInterceptor {
+
+        @Override
+        public void aroundWriteTo(WriterInterceptorContext context) throws IOException, WebApplicationException {
+            context.proceed();
+        }
+    }
+
+}