Inherited Tenant and Supposed Last Months Rent Deposit

[Tim Graham]

First investment property! Duplex and I'm house hacking. That means one of the tenants needs to go so I can move in. Unfortunate, but necessary scenario.

The tenant has not paid this months rent (their last month) claiming that 9 years ago and 2 landlords ago the original lease included a payment of first and last months rent as well as a security deposit. Per the current lease - the only lease I have ever seen - the only moneys on file for them is the $500 security deposit which did transfer at closing.

They are going to look for a copy of their old lease, but even if they find it, am I bound legally or ethically to honor that old lease? Or do they still owe me this months rent?  Any help would be great. The property is located in Washington State in case that matters.