Yes, these are inherent properties of browsers. When you post something and then press the back button, the form gets reposted. This is the reason that in e-commerce sites when you submit a payment, the next page says "do not press the back button".
.
There are some complicated workarounds to this issue, but we have chosen to keep it simple. We do try to prevent the creation of duplicates in some cases (but not in all cases).