In the past 3 years or so, I've probably though "spy" 3 or 4 times. Based on what I was in or where I was... But at the same time, I have "bumped" into random goons or NOE bombers just as many times. It's just one of those things. It probably exists but I don't think much.
How often does this happen? It can't be that often.
You're serious aren't you? Whoa...

All in all, I don't think alerting the player they have a wingman is a bad idea at all. That way, now you know the callsign, you also know the enemy player, hit R, take a screeny of the wingman, send it all into HTC. They can then see where the 2 callsigns are coming from and take appropriate action. I will only speak for myself when I say, I do believe that if a person is using 2 accounts to hone in on one player specifically, should be looked at as cheating.
At the same time the easy, although annoying, fix for this is to simply switch fighting that country. The other person will change countries to go fight you again. Now you can switch back and have and hour & half of no "person" around.

I can absolutely understand the suggestions of paranoia. Only those that have experienced this first hand can tell you just how creepy some players can be, and the lengths they will go to. Relogging, switching to the third country, taking small breaks, flying different aircraft from your favourite one, having a friend fly your favourite one near you in your paint scheme while you fly his in his paint scheme, and even doing all of this for hours on end will not work to deter a dedicated griefer. He can find you immediately at any time in any furball, any group of winging aircraft, or in any place you go to alone on the map regardless of what aircraft you are flying.