improve performance of backfill avoid queuing more than 1 backfill call

app/assets/javascripts/discourse/models/post-stream.js.es6

@@ -732,11 +732,16 @@ export default RestModel.extend({
     this._excerpts = this._excerpts || [];
     const stream = this.get('stream');
 
+    this._excerpts.loadNext = streamPosition;
+
     if (this._excerpts.loading) {
       return this._excerpts.loading.then(()=>{
         if(!this._excerpts[stream[streamPosition]]) {
+
+          if (this._excerpts.loadNext === streamPosition) {
             return this.backfillExcerpts(streamPosition);
           }
+        }
       });
     }