Tracking down lost evidence of paid debt

so i paid off a debt with a collector i think back in 2007 or 2008 in which they sent me a letter say my debt was paid and cleared. it was for a car loan originating honda finance. then couple years back, some collector name rpm sends me a letter to collect in which i sent them a letter saying that i've already cleared and paid, and stop harrassing me and to clear my records of this debt. never heard from them after that. then today, i've received a letter from another agency trying to collect?
whats going on here? i've already paid and cleared out my debt. however, i don't have the original letter from the agency that i've paid cleared my debt with. how do i track down and prove that i've paid my debt? are these guys just trying to attempt to see if im dumb and pay? how do i stop them?