This error can occur in two cases:
1. Client wants to edit his address but click on save button several times. This creates duplicity in database and client cannot make another order due to the carriers error.
2. Client who already made order before, changes the address. Then he can't make another order.
This problem occurs in 1.7 versions, including latest 126.96.36.199. The fixes should come probably in 1.7.7. If you want to fix it in older versions, we will do it manually for you. The price is 20 EUR.