If second back up makes you feel better, then there's nothing wrong with that. However, the IUD is designed so that you don't have to use anything. That's the beauty of it - freedom!
You will find stories of women who became pregnant with an IUD, just as you will find women who become pregnant even when taking their bcp regularly. The chances, though, are quite low - don't over-estimate them. For 99% accuracy, if you have sex 100 times, only ONCE do you run any sort of risk.