This Mk7 replacement features a spring-tensioned, quick-release 623 bearing idler, and won't interfere with dualstrusion. The back is also open to make it easy to clean the teeth of the drive gear.
This works with MBI's Mk7 and QU-BD's MBE extruders. It will fit the Mk8 as well, but the Replicators have plates above the extruders that will interfere with the spring arm. There are several derivatives that solve this problem, most at the expense of the rear clean-out opening.
The best shaft for the bearing seems to be an M3x10mm flat-head screw, inserted from the motor side of the idler piece. A piece of 3mm filament should also work in a pinch.