Fix Redis Iterators (#318)

Redis SCAN may return same item multiple times, add a basic check to avoid duplicate entries.

storage/redis/client.go

@@ -151,12 +151,12 @@ func (client *Client) Iterate(opts storage.IterateOptions, fn func(it storage.It
 
 func (client *Client) allPrefixedItems(prefix, first, last storage.Key) (storage.Items, error) {
 	var all storage.Items
+	seen := map[string]struct{}{}
 
 	match := string(escapeMatch([]byte(prefix))) + "*"
 	it := client.db.Scan(0, match, 0).Iterator()
 	for it.Next() {
 		key := it.Val()
-
 		if first != nil && storage.Key(key).Less(first) {
 			continue
 		}
@@ -164,6 +164,11 @@ func (client *Client) allPrefixedItems(prefix, first, last storage.Key) (storage
 			continue
 		}
 
+		if _, ok := seen[key]; ok {
+			continue
+		}
+		seen[key] = struct{}{}
+
 		value, err := client.db.Get(key).Bytes()
 		if err != nil {
 			return nil, err