Question pretty much says my dilemma. Wallet won't load due to a "duplicate transaction" which I thought could not happen. I use multibit btw, if that helps. Please tell me I can load my wallet somehow.
This was a bug that was fixed in v0.4.1 (26 June 2012) so I presume you are running an earlier version. If you download the latest version from multibit.org your wallet should load ok.
Sorry for the inconvenience.