//On top of script
new objwindow; //Store the window object in this variable

//Somewhere in script - objwindow = CreateObject(); //Create your window object.

public OnPlayerWeaponShot(playerid, weaponid, hittype, hitid, Float:fX, Float:fY, Float:fZ)
{
    //If the player hit an object, and the object ID that was hit is our window 'objwindow'
    if(hittype == BULLET_HIT_TYPE_OBJECT && hitid == objwindow) return 0; //Return 0 to avoid damage
    else return 1;
}

//On top of script
new objwindow; //Store the window object in this variable

objwindow = CreateObject(); //Create your window object.

public OnPlayerWeaponShot(playerid, weaponid, hittype, hitid, Float:fX, Float:fY, Float:fZ)
{
    //If the player hit an object, and the object ID that was hit is our window 'objwindow' it will create the window again.
    if(hittype == BULLET_HIT_TYPE_OBJECT && hitid == objwindow) return objwindow = CreateObject; // Type the same function you used in creating the object the first time.
    return 1;
} //Credit to ABKPot of course.