"Helicopter Event": {
      "Enabled": true,
      "Display Name": "Helicopter",
      "Disable Vanilla Event": true,
      "Event Start Offset (Minutes)": 0.0,
      "Minimum Time Between (Minutes)": 60.0,
      "Maximum Time Between (Minutes)": 180.0,
      "Minimum Online Players Required (0 = Disabled)": 0,
      "Maximum Online Players Required (0 = Disabled)": 0,
      "Announce Next Run Time": false,
      "Restart Timer On Entity Kill": true,
      "Kill Existing Event On Plugin Loaded": true,
      "Maximum Number On Server": 1,
      "Exclude Player's Entity": true,
      "Normal Event Weight (0 = Disable)": 60.0,
      "HeliRefuel Plugin Event Weight (0 = Disable)": 20.0,
      "PilotEject Plugin Event Weight (0 = Disable)": 20.0
    }

also sometimes the helicopter spawns right again after it was shot down. 
i thought

"Restart Timer On Entity Kill": true​

should fix that.

If it is on the server the maximum number is 1, it should not have two, it could have been spawned by another plugin

ok. but the

Restart Timer On Entity Kill​

doesnt seem to work. we had it multiple times that the heli respawn right after it got killed

Check the server console for debug messages when the helicopter is spawned.