Do you mean, concentrating sunlight onto PV? or Having PV cells that are also reflective?

Reflective PV cells/mirrors wouldn't work well as you're taking the energy out one way or another. The more you reflect, the less you can use for PV, and vice-versa.

Sorry for being unclear. I meant concentrating sunlight onto existing PV panels. I suppose that I'm assuming mirrors << PV in terms of cost.

I've heard of some folks using a Fresnel lens to concentrate light onto a smaller area, where the silicon is. It improves the efficiency significantly, but can also cause heat issues (which can hurt efficiency).

I've tried this. With standard PV panels going over 2:1 is not advisable. The panels will live a lot shorter and if you go higher than 2:1 they will break (wiring will de-solder, the plastic bits & pieces will melt and they sometimes simply stop working). I've tried watercooling as well (with very thin panels) and that works better but the lifespan reduction is still significant, you can see month-to-month performance degradation (as compared to a control that is running on 1 sun).

Latest PV kinda use mirrors internally