From what I have read, keys in dictionaries are unique.
Consider this code:
NSMutableDictionary *mydic = [NSMutableDictionary dictionary];
[mydic setObject:@"value1" forKey:@"key1"];
[mydic setObject:@"value1" forKey:@"key1"];
[mydic setObject:@"value1" forKey:@"key1"];
Why can I run this without any error? What should I do to avoid duplicate keys?