Merge pull request #994 from initlifeinc/master

duplicate failed urls are added into failedURLs

@@ -194,10 +194,12 @@
 
 
                     if (error.code != NSURLErrorNotConnectedToInternet && error.code != NSURLErrorCancelled && error.code != NSURLErrorTimedOut) {
                     if (error.code != NSURLErrorNotConnectedToInternet && error.code != NSURLErrorCancelled && error.code != NSURLErrorTimedOut) {
                         @synchronized (self.failedURLs) {
                         @synchronized (self.failedURLs) {
+                            if (![self.failedURLs containsObject:url]) {
                                 [self.failedURLs addObject:url];
                                 [self.failedURLs addObject:url];
                             }
                             }
                         }
                         }
                     }
                     }
+                }
                 else {
                 else {
                     BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);
                     BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);