Today it happened that I had placed on G4 on the map.
The other team later unaware of my mine also put a mine on G4.
Situation 2 mines on G4.
In the next turn I actived the mine and did 1 damage to the oppossing submarine. And wondered what happened to the other mine..that hypothetically could be on that exat same location.
We assumed that if there was another mine on G4 that this was also destroyed in the explosion. The rulebook has no ruling on this.
Also it is unclear if the other team should mention that their mine was destroyed at the same time...or that they simply cross it out on their sheet and don't tell the other team.
Hi, the other team's mine wont be destroyed by this, the only way to destroyed a mine without activate it is hitten by a torpedo.
For your situation, there is nothing affect to the other team's mine.