It's very anoying to spend half your ammo riping a plane apart and when he is just about to go down, one of your team mates finishes him out and claims the full kill! It's stupid and I know that in REAL war kills were shared. You could get half a kill!
I agree, I've had this happen to me more than once. A common solution is to get your wingmen to target a different aircraft or ground unit or just make them form up.