Valve cover gasket replacement

Not sure how to correct this in ur system but my diesel's milage is 270,000 not 27,000, my mistake.
My question is: if you take off a relatively new valve cover gasket and put it back on can you safely use the same gasket or does the gasket need to be replaced with a new one each time you take it off?
I am using a shop I have trusted for ten years who now has a new mechanic. The truck had a metal on metal sound when first starting the motor. He replaced one gasket November 2016. And did the other side last week. They said they they could not find anything wrong with the one just opened so they opened the one done in November to see if there was a problem there. Nothing there either so the put same gasket back on (the one done in November) and the new gasket on the other side and it ran fine. And he put Blue Devil in my truck. I do not use any additive stuff ever. Not sure why he would put it in unless he damaged the gasket done in November when opening it for second time. I am on a fact finding mission here. Just doing my research before I talk with my old friend the shop owner. Any help will be appreciated.