Paper cut: Check for duplicates

Checking for duplicates would be a very useful feature here. Again, I am focusing on suggesting business-friendly improvements to this package, to keep the tracking list clean. Therefore, the package should notify the user that they tried to enter a duplicate tracking number. Assuming this gets implemented around the same time as a "fix" for 1476314, it would be good to pop open a dialogue box saying:
   The following tracking numbers:
    RP111111111SG
    RP111111112SG
    RP111111113SG
    Are already on your tracking list and were not entered.

Another option would be to add a dialogue asking the user whether they want for the new entries to replace the older ones or to skip entering them. I just figured the original suggestion is easier to implement, but if this is trivial.....