Sorry replying late,
Grump642, there has been an awful wind, actually some sort of hurricane over parts of England, London included and some branches from a very tall tree next to our building broke the window of my apartment. So, a lot of unexpected things to do, lol, but all has been taken care of now. So, now the game: When you finish the Seriy protection job and you speak with him, all hell breaks loose (if you don't count the previous Horror time and the bandits, lol)
If you don't speak with Seriy, then the mission will remain incomplete, so when you speak with him, a couple of
info-portions are released (such info will be checked and required by other following events and missions) and you get the
Groza_NT +missile/grenades and also the infamous
Garbage Heli (with a
different very aggressive logic than the others, a logic that some other Helis-
that come only in Extreme mode- also use in various parts of the game) and the
special Extreme mode APC are also released... all at he same time.
The thing is, this Garbage Heli
above Seriy's Hangar(as it has become famous so) is not the only Heli in Garbage! There's also another resident Heli from the normal NEWSA.SIMBION_T files, which appears randomly depending on players progress and a bunch of other checks. So it seems you have validated the appearance of the first/resident/universal Heli (call it whatever you like

) and also the EXtreme Mode Garbage Heli has been summoned as well. Great!

It rarely happens, randomness (though is not a preset value) or better to say the chances are low; so you've hit the jackpot. Sorry.
When you re-enter Garbage from the Military base side after you've completed the "get the military case" mission, the X_Mode Garbage Heli (that notorious one) and the X-Mode Tweaked APC are both re-spawned again. However, it has come to my attention that, on some CPU/RAM configurations, this APC may not re-spawn the second time. (I had spoke with someone from Intel on this and matters like these once, and the answer was -somewhat vaguely- that it was just one of those things. Yeah, I think it wsa the right answer

)
I remember that this double Heli attack had happened to me at least once, though not quite often, either. But, the funny thing is that I don't remember what the hell did I do about it, lol.

If I had my stubborn side at that time, I might've try and try and try until I passed out, lol; or if it was a lazy side of me that was in front of the PC, then I might've reloaded from an earlier save. Or, you can try keep sleeping for 3 hours each time one after another, eventually game will start throwing out different scenarios, Horror-Times, etc, and that Heli routine will divert.