If an Imperiused wizard, like a Death Eater, was Imperiused by an Auror, would it override the control the first wizard had over the Death Eater, like say, Voldemort?
I would say, it all depends on the power of the wizards who casten the Imperius. The most powerful wizard gains or keeps control over the target.
In your example, the Imperiused man would more likely stay under Voldemort's control since there are a few wizards who are more powerful than him.
Then, what if Harry or Dumbledore casted an Imperius too ? Assuming that one is stronger than Voldemort, he would win and gain control, but if their strength were equal, then the target might not be able to withstand that mental battle and die, if the 2 wizards keep castening Imperius with equal strengths.
It remains an assumption, I mean, no one told that, but I rely on the fact that we already know Imperius can affect the mind (cf. Herbert Chorley who became a bit crazy and was sent to the Hospital for Magical Maladies). Considering that, we can admit that a mental battle for someone's mind which would go on for too long could damage his mind more and more until death.