Never got double on amazon, did once get someone else's packing slip in my box.
I did recently got a duplicate order of a Yamaha iPod speaker from Vault.
Also recently sears canceled an order of mine (with no reason given), gave a refund, and I received the order anyway.
Guess you can add if the extra item you received was just an extra item thrown in by the packer, or did the system duplicate your order and you received 2 seperate packages.