diff --git a/README.md b/README.md
index df82aacb..65411360 100644
--- a/README.md
+++ b/README.md
@@ -193,7 +193,7 @@ Common Problems
 
 ### Using dynamic image size with UITableViewCell
 
-UITableView determins the size of the image by the first image set for a cell. If your remote images
+UITableView determines the size of the image by the first image set for a cell. If your remote images
 don't have the same size as your placeholder image, you may experience strange anamorphic scaling issue.
 The following article gives a way to workaround this issue:
 
diff --git a/SDWebImage/SDImageCache.m b/SDWebImage/SDImageCache.m
index 36bb751a..dbc2d969 100644
--- a/SDWebImage/SDImageCache.m
+++ b/SDWebImage/SDImageCache.m
@@ -48,7 +48,6 @@ BOOL ImageDataHasPNGPreffix(NSData *data) {
     static id instance;
     dispatch_once(&once, ^{
         instance = [self new];
-        kPNGSignatureData = [NSData dataWithBytes:kPNGSignatureBytes length:8];
     });
     return instance;
 }
@@ -61,6 +60,9 @@ BOOL ImageDataHasPNGPreffix(NSData *data) {
     if ((self = [super init])) {
         NSString *fullNamespace = [@"com.hackemist.SDWebImageCache." stringByAppendingString:ns];
 
+        // initialise PNG signature data
+        kPNGSignatureData = [NSData dataWithBytes:kPNGSignatureBytes length:8];
+
         // Create IO serial queue
         _ioQueue = dispatch_queue_create("com.hackemist.SDWebImageCache", DISPATCH_QUEUE_SERIAL);
 
diff --git a/SDWebImage/SDWebImageManager.m b/SDWebImage/SDWebImageManager.m
index 38f75f0c..0d2f31be 100644
--- a/SDWebImage/SDWebImageManager.m
+++ b/SDWebImage/SDWebImageManager.m
@@ -194,7 +194,9 @@
 
                     if (error.code != NSURLErrorNotConnectedToInternet && error.code != NSURLErrorCancelled && error.code != NSURLErrorTimedOut) {
                         @synchronized (self.failedURLs) {
-                            [self.failedURLs addObject:url];
+                            if (![self.failedURLs containsObject:url]) {
+                                [self.failedURLs addObject:url];
+                            }
                         }
                     }
                 }