>I have a mac that I never use, would storing my bitcoins on a wallet on that laptop and then only connecting it to the internet when I am ready to transfer money be safe? Is there any reason not to do this?

Your laptop needs to catch up every time you want to make a transaction, which is not good.

The ideal solution would be Bitcoin Armory, which is designed for this type of thing:

http://bitcoin.stackexchange.com/questions/8101/how-to-sign-a-transaction-using-only-offline-computer

[Find the mac version here](https://bitcointalk.org/index.php?topic=156250.msg1655941#msg1655941)

>Would I get an error from the P2P network or would I have duplicate coins 

The second time you spend the coins, other nodes will reject it.