Just so you know, you can't do this with Monteblanc or your starter guy. For this to work, you need two people and a link cable.

Key:

Clan 1 = person who wants two of the same character
Clan 2 = person who is helping the duplication
Jerry = member of Clan 1 who is being duplicated

First, Clan 1 save your account in both slots. Clan 2 needs to have a lot of gil. Then, Clan 1 trade Jerry to Clan 2. Now, Clan 1 switch to the account that DOES have Jerry. Next, Clan 2 trade your version of Jerry to Clan 1. If done correctly, Clan 1 should have two Jerrys

Note: If you save something wrong, then you will be totally messed up and will not have Jerry anymore.