_aiUnits = []; /// Define a list (Array) 
{
    if (!isPlayer _x) then { /// If _x (counter, part of foreach loop) is not player

          _aiUnits pushback _x; /// add _x to _aiUnits list. We need this array later.
          
      };
} forEach allUnits;  /// For each units which has been placed on the map

{
       _x addEventHandler ["HandleDamage",  /// Add HandleDamage Event Handler to all AI units which we have assigned in _aiUnits
        { 
            _unit = _this select 0;    /// _unit is the same as _this select 0 (_this select 0 = unit: Object - Object the event handler is assigned to. Source = https://community.bistudio.com/wiki/Arma_3:_Event_Handlers#HandleDamage)
            _shooter = _this select 3, /// this select 3 is the source which caused the damage, so we call it _shooter
            
            if (_shooter isEqualTo player) then {        /// here we check if the shooter is equal to the actual player
        
                                                        /// I did it this way, so you can assign a command to what should happen when player hit an AI 
            }
            else {

                _unit setDamage 0;                        /// If AI hits another AI we set the damage to 0 which means the unit is fully "repaired"
                
            }
        }
    ];
} forEach _aiUnits;                                        /// Here we have a forEach loop. Foreach _aiUnits do this.

if (!isServer) exitWith {};

private _aiUnits = [];
   {
	if (!isPlayer _x) then {
        _aiUnits pushback _x;
    };
    true
} count allUnits;

{
    _x addEventHandler ["HandleDamage",
        {
            _unit = _this select 0;
	    _damage = _this select 2;
            _shooter = _this select 3;
            
            _return = if (isPlayer _shooter) then {
		0
            } else {
                _damage
            };
	   _return
        }
    ];
    true
} count _aiUnits;