If you already have Sets handy - why not use the Set Intersection? (Assuming the Set implementation has that capability).

Putting both contents (lists, arrays, whatever you have) into sets, and then calculating the intersection might be more expensive than building the intersected set right away sourcing both non-set contents, I imagine. Though it would probably be easier to read and understand. But that can be solved by naming of functions that process the 2 non-set things.