Actually, I don't think that's quite correct, as you can't "rebuild" capitals, at least in DA and DL, to prevent the exploit of building a capital, giving the planet it's on to an AI, then rebuilding it elsewhere and taking the planet back to have multiple capitals.

However, if you conquer a planet where a capital or another "unique" project is under construction, it will still finish even if you've already built your own version of that particular capital. Furthermore, capitals built on worlds you conquer aren't destroyed even if you've already built one. Perhaps the Drengin have just been very lucky in the planets they've conquered.

If they haven't conquered any planets at all, you're most likely experiencing some kind of a bug.

Each race can only begin construction on one capital. Once it's under construction (or completed), anyone taking that planet gets to keep it, and it will be fully functional. It's not like the Econ captial is all that overpowered anyway, and the AI tends to put it on sub-optimum planets.

I've personally farmed logistics buildings to the point of getting 10 huge hulls in one fleet. It can be done.