The left one is the idler with the jrsc kit. Good luck getting a hold of a new one. The right one is the stock idler which you can get a new one from any auto parts store.
I had the same problem. Replaced the tensioner and belt and it took care of it.
04 ZX3 Mostly Stock [3/13 ROTM]["Best Non-SVT" FITF '12][7/14 ROTM]
12 SE Hatch Sport & Winter Package, ST Front End
94 F150 XLT DD
89 Bronco II XLT Well the body is still stock, what's left of it