More information is needed. What is the landlord trying to charge you for, now? Was there anything that was not paid, before? While it does not seem fair, it is likely that you and any other roommates were jointly and severally liable for any and all charges.
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Usually tenants are jointly and severally liable for the rent. So the landlord can collect it all from you and you have top go after them. However if it was paid you have the defense of "accord and satisfaction" or Res judicata.
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