Think the May 12th patch [xcom.com] fixed this:
Mimic Beacons can no longer be reanimated

Hi, thanks for doing this. Does this also fix the "friendly fire" warning if you're using explosives/AoE on the expired mimic beacon's location?

@Yamino Yeah, seems pretty common to me too. However, I've also got a mod that adjusts how often the Sectoids use Zombie, and that definately helps. No longer means it's totally safe to ignore sectoids in their first turn. None the less, should apply this mod too to further reduce the OP Mimic Beacons (and prevent the need to nerf them even further)

I had it happen to me just once and laughed out loud when it did, that ridiculous it was.

@Warbadger: While I managed to do so the first time I used a Mimic Beacon. It's pretty random, since it matters where you use it and how...but considering how Sectoids love their ressurrection skill...I'd say if you have a mimic beacon out and a living Sectoid, the probability isn't low. (actually happened for me everytime I had that scenario)

@Yamino To be fair, I've never encountered this little bug myself in all the hours I've played...

And another bugfix that had to be made by the modders. Thank you.