IOS How to use viewWithTag and addSubview 

Move the code [self.view addSubview:label]; inside your if
 block.  When your if condition is false, that means the label is 
already part of of your viewcontroller's view hierarchy, so if you add 
it again like in your original code it will be double retained 
UILabel *label = (UILabel *)[self.view viewWithTag:100];
if (!label) {
    label = [[[UILabel alloc] initWithFrame:CGRectMake(0, 0, 20, 20)] autorelease];
    label.tag = 100;
    [self.view addSubview:label];
}

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