I filed bankruptcy one year ago; now I have received a summons from one of the creditors. I had 20 days to answer them; subsequently, I sent them a letter along with my discharge papers stating they were listed as one of the creditors.

Should I contact an attorney for this matter?

No need too. If you have copies of your discharge papers showing proof that you have been discharged from that debt specifically. They do not have any grounds against you.

If they continue, then I would contact whoever you dealt with your bankrutpty. Just make sure that you still have those papers of your discharge for yourself. That is your proof. Hope you made copies or were giving extra copies of discharge as proof.