diff --git a/src/t_zset.c b/src/t_zset.c
index ab094654..f5f2d7b5 100644
--- a/src/t_zset.c
+++ b/src/t_zset.c
@@ -1981,51 +1981,78 @@ void zunionInterGenericCommand(redisClient *c, robj *dstkey, int op) {
             zuiClearIterator(&src[0]);
         }
     } else if (op == REDIS_OP_UNION) {
+        dict *accumulator = dictCreate(&setDictType,NULL);
+        dictIterator *di;
+        dictEntry *de;
+        double score;
+
+        if (setnum) {
+            /* Our union is at least as large as the largest set.
+             * Resize the dictionary ASAP to avoid useless rehashing. */
+            int minlen = setnum ? zuiLength(&src[setnum-1]) : 0;
+            dictExpand(accumulator,minlen);
+        }
+
+        /* Step 1: Create a dictionary of elements -> aggregated-scores
+         * by iterating one sorted set after the other. */
         for (i = 0; i < setnum; i++) {
-            if (zuiLength(&src[i]) == 0)
-                continue;
+            if (zuiLength(&src[i]) == 0) continue;
 
             zuiInitIterator(&src[i]);
             while (zuiNext(&src[i],&zval)) {
-                double score, value;
-
-                /* Skip an element that when already processed */
-                if (dictFind(dstzset->dict,zuiObjectFromValue(&zval)) != NULL)
-                    continue;
-
-                /* Initialize score */
+                /* Initialize value */
                 score = src[i].weight * zval.score;
                 if (isnan(score)) score = 0;
 
-                /* We need to check only next sets to see if this element
-                 * exists, since we process every element just one time so
-                 * it can't exist in a previous set (otherwise it would be
-                 * already processed). */
-                for (j = (i+1); j < setnum; j++) {
-                    /* It is not safe to access the zset we are
-                     * iterating, so explicitly check for equal object. */
-                    if(src[j].subject == src[i].subject) {
-                        value = zval.score*src[j].weight;
-                        zunionInterAggregate(&score,value,aggregate);
-                    } else if (zuiFind(&src[j],&zval,&value)) {
-                        value *= src[j].weight;
-                        zunionInterAggregate(&score,value,aggregate);
+                /* Search for this element in the accumulating dictionary. */
+                de = dictFind(accumulator,zuiObjectFromValue(&zval));
+                /* If we don't have it, we need to create a new entry. */
+                if (de == NULL) {
+                    tmp = zuiObjectFromValue(&zval);
+                    /* Remember the longest single element encountered,
+                     * to understand if it's possible to convert to ziplist
+                     * at the end. */
+                    if (sdsEncodedObject(tmp)) {
+                        if (sdslen(tmp->ptr) > maxelelen)
+                            maxelelen = sdslen(tmp->ptr);
                     }
-                }
-
-                tmp = zuiObjectFromValue(&zval);
-                znode = zslInsert(dstzset->zsl,score,tmp);
-                incrRefCount(zval.ele); /* added to skiplist */
-                dictAdd(dstzset->dict,tmp,&znode->score);
-                incrRefCount(zval.ele); /* added to dictionary */
-
-                if (sdsEncodedObject(tmp)) {
-                    if (sdslen(tmp->ptr) > maxelelen)
-                        maxelelen = sdslen(tmp->ptr);
+                    /* Add the element with its initial score. */
+                    de = dictAddRaw(accumulator,tmp);
+                    incrRefCount(tmp);
+                    dictSetDoubleVal(de,score);
+                } else {
+                    /* Update the score with the score of the new instance
+                     * of the element found in the current sorted set.
+                     *
+                     * Here we access directly the dictEntry double
+                     * value inside the union as it is a big speedup
+                     * compared to using the getDouble/setDouble API. */
+                    zunionInterAggregate(&de->v.d,score,aggregate);
                 }
             }
             zuiClearIterator(&src[i]);
         }
+
+        /* Step 2: convert the dictionary into the final sorted set. */
+        di = dictGetIterator(accumulator);
+
+        /* We now are aware of the final size of the resulting sorted set,
+         * let's resize the dictionary embedded inside the sorted set to the
+         * right size, in order to save rehashing time. */
+        dictExpand(dstzset->dict,dictSize(accumulator));
+
+        while((de = dictNext(di)) != NULL) {
+            robj *ele = dictGetKey(de);
+            score = dictGetDoubleVal(de);
+            znode = zslInsert(dstzset->zsl,score,ele);
+            incrRefCount(ele); /* added to skiplist */
+            dictAdd(dstzset->dict,ele,&znode->score);
+            incrRefCount(ele); /* added to dictionary */
+        }
+        dictReleaseIterator(di);
+
+        /* We can free the accumulator dictionary now. */
+        dictRelease(accumulator);
     } else {
         redisPanic("Unknown operator");
     }