How to mirror a complete part?

I am doing a project that has a lot of small metal parts, some of which require "opposites."  Think of a part shaped like an "L" and then needing a backwards "L" to go with it.  These parts are small, but they have a ton of different holes all over them; is there a fast way to produce a mirror of a complete part without having to physically model it from the ground up?  Thanks in advance for any replies!

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