Quoted from Kawydud:

Has anyone else experienced two balls being ejected into the shooter lane at times? We have a game out on route that sometimes kicks two balls out at once. It isn't a switch problem where it double ejects, it is actually two balls coming out at once.

Quoted from pinnyheadhead:

Is one ball sticking to another because they are magnetized? Switch out the balls and try.

Quoted from pinnyheadhead:

Balls can weirdly become magnetized. Who knows?

Quoted from pinballaddicted:

There is a metal tab on the eject side (left) of the shooter. If you bend it a little, so the ball bounces of the tab and in to the lane, the ball will stop bouncing straight back in to the shooter trough. This is common on new Sterns. What happens, the first ball is ejected in to the shooter lane, and the next ball takes its place. The first ball bounces back in to the shooter so now there are two balls there on top of each other and no ball in the shooter lane. Machine ejects two balls out. B/Ws had the same issue.
Good luck!