I co-signed for a lease on November 7. The party decided not to take the apartment and no lease was signed. I assumed my liability as co-signer was null and void. On December 9th, without my knowledge the party went back to the company and signed a lease, for the same unit. The management company now says I am the liable co-signer. I did not see the lease prior to it being completed nor is a co-signer mentioned on the lease. The management company claims my co-signer agreement is an "addendum" to the lease. We live in Washington State. Can they hold me liable?

You co-signed... so as long as that lease is in effect... you are liable.

That's why you always need to think twice before you cosign for anything for anyone.

They used the same paperwork... as it was for the same unit. The original was never torn up... only had a delayed signing. One month + 2 days.

This will be a court issue, You need to notify the apartment in writing in certified mail, that the contract was voided when the renter decided not to take the apartment. So after that, when they decided to rent again, this was new and not part of your signing.

Now you will not know what will happen till sometime does happen. Of course the apartment will claim you liable, so as long as the person pays all of the rent, nothing happens, but if they end up owing, of course you will be sued, so that will be your defense.