do not allow to extend same field twice to prevent the error

.gitignore

@@ -10,3 +10,4 @@ coverage/
 sandbox/
 .nyc_output
 dist/
+.gitpod.yml

src/root.js

@@ -273,6 +273,10 @@ function tryHandleExtension(root, field) {
     var extendedType = field.parent.lookup(field.extend);
     if (extendedType) {
         var sisterField = new Field(field.fullName, field.id, field.type, field.rule, undefined, field.options);
+        //do not allow to extend same field twice to prevent the error
+        if (extendedType.get(sisterField.name)) {
+            return true;
+        }
         sisterField.declaringField = field;
         field.extensionField = sisterField;
         extendedType.add(sisterField);

tests/comp_import_extend.js

@@ -0,0 +1,18 @@
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+var path = require("path");
+var tape = require("tape");
+var protobuf = require("../index");
+// to extend Root
+require("../ext/descriptor");
+tape.test("extensions", function (test) {
+    // load document with extended field imported multiple times
+    var root = protobuf.loadSync(path.resolve(__dirname, "data/test.proto"));
+    root.resolveAll();
+    // convert to Descriptor Set
+    var decodedDescriptorSet = root.toDescriptor("proto3");
+    // load back from descriptor set
+    var root2 = protobuf.Root.fromDescriptor(decodedDescriptorSet);
+    test.pass("should parse and resolve without errors");
+    test.end();
+});

tests/comp_import_extend.ts

@@ -0,0 +1,35 @@
+import path = require("path");
+import * as tape from "tape";
+
+import * as protobuf from "../index";
+import { IFileDescriptorSet } from "../ext/descriptor";
+// to extend Root
+require("../ext/descriptor");
+
+interface Descriptor {
+  toDescriptor(
+    protoVersion: string
+  ): protobuf.Message<IFileDescriptorSet> & IFileDescriptorSet;
+  fromDescriptor(
+    descriptor: IFileDescriptorSet | protobuf.Reader | Uint8Array
+  ): protobuf.Root;
+}
+
+tape.test("extensions", function (test) {
+  // load document with extended field imported multiple times
+  const root = protobuf.loadSync(path.resolve(__dirname, "data/test.proto"));
+  root.resolveAll();
+
+  // convert to Descriptor Set
+  const decodedDescriptorSet = (root as unknown as Descriptor).toDescriptor(
+    "proto3"
+  );
+
+  // load back from descriptor set
+  const root2 = (protobuf.Root as unknown as Descriptor).fromDescriptor(
+    decodedDescriptorSet
+  );
+
+  test.pass("should parse and resolve without errors");
+  test.end();
+});