When this happens look at their combat rank. If it is anything other than elite, they may be farming combat XP, in which case they have to decide if half the combat XP that they can make working alongside you, is faster than shooting stuff on their own. I don't often run into other players that can take out NPCs as efficiently as I, and that's not a brag, I spend all my time in combat, I don't know the first thing about trading, so it's only logical, therefore I would rarely accept a wing invite in a RES, because I value my kills for my combat rank.
Generally, if I find myself competing for kills, one of two things would happen based on an assessment of the other player...Do I believe this person is deliberately stealing my kills? If so, then they get a warning, then they get shot. If I think it's genuine competition I will try to stay clear of the other pilot and focus on my own kills. If that turns out to be impossible, there's always another RES.
Another thing is I also have small kids and a demanding wife, and it can be annoying for others in a wing that I go AFK every 5 minutes for 1 or 2 minutes. Basically, I wing up if I've got definite free time ahead o me, if there's a chance I'll be interrupted I won't accept the invite, but if I decline, I would alsways explain myself ("playing casual, may need to go afk often, will just be a pain in your rear, but thanks for the offer")