Exchange Email Changes (Obstacle)

I have run into a pretty big obstacle with my email change script. So right now I did some testing and I am able to assign the same smtp address to multiple people. I think it is a replication issue that I am running into and so I am not getting an error stating that a smtp address is in use when I set-mailbox. So in order to make sure that I am not going to have multiple accounts with the same address I am going to go get all the smtp addresses from every mailbox and dump that to a file. Once it is in the file I will complare any new addresses that I need to create to that list and verify that I am not duplicating. Every time I add a new address I will also have to update that file so that I can have an up to date list of what addresses are in use before I create the next address. This is not to hard to do just a step that I was hoping to avoid.