I was victim to the double attack bug once. It happens. The worst is when some one loses something because of that bug, they get it back, and the person who won the other one keeps it too, effectively duplicating the planet or cluster. lol
The double attack bug is rare but does happen once in awhile. FourtyThree is correct on posting on the help center.
Also their is a rare x2 attack bug where you can get hit by two empires at once if done at the same time. GG got a 18k large from me like that once but it was granted back to due to being a bug.