Don’t ignore it; it’s best to take action rather than hope it will go away. Send the letter contesting the agency’s claim with any proof that the bill was paid (if you have any). Keep a copy of all correspondence and take notes on any phone conversations you have with the agency (and ALWAYS get the name of the person you are speaking with). As Linda says, if truth is on your side, it’s a battle you should be able to win.
Also, check your credit report if you haven’t already done so. Every twelve months, you can get a free copy of your report from all three credit agencies at www.annualcreditreport.com. If the debt shows up on any of those reports, you’ll need to alert the agencies and go through the process of having it removed.
Still, as it has been discussed on here lately, it may be a trick. If anything call Chase directly.