Fix module ID deduplication (#20406)

Author: gaearon

Diff for: packages/react-server/src/ReactFlightServer.js

@@ -432,7 +432,8 @@ export function resolveModelToJSON(
     if (isModuleReference(value)) {
       const moduleReference: ModuleReference<any> = (value: any);
       const moduleKey: ModuleKey = getModuleKey(moduleReference);
-      const existingId = request.writtenModules.get(moduleKey);
+      const writtenModules = request.writtenModules;
+      const existingId = writtenModules.get(moduleKey);
       if (existingId !== undefined) {
         return serializeByValueID(existingId);
       }
@@ -444,6 +445,7 @@ export function resolveModelToJSON(
         request.pendingChunks++;
         const moduleId = request.nextChunkId++;
         emitModuleChunk(request, moduleId, moduleMetaData);
+        writtenModules.set(moduleKey, moduleId);
         if (parent[0] === REACT_ELEMENT_TYPE && key === '1') {
           // If we're encoding the "type" of an element, we can refer
           // to that by a lazy reference instead of directly since React