Recently, I had the interesting situation that a hero got poisoned at the start of his turn. I assume that as the Start of Turn phase was not over yet, he needed to test for Poisoned.

However, what would happen in the following situation:
A hero is poisoned, diseased and fully fatigued while in the Secret Room Tainted Spring. This room has the effect: Each time a hero suffers any amount of damage while on this room is also poisoned."

Let's say at the start of his turn, the hero chooses to test for Poisoned first and succeeds. Then he tests for Diseased and fails. He suffers 1 damage and is poisoned again from the Secret Room effect. Does he have to test for Poisoned again, as it is a new instance of being Poisoned? Of course, he could have tested for disease first to prevent this, but let's say he did not.

From my point of view its like OL with the Dark Charm card being his last Deck Card and all other cards in his hand.

Start of Turn
OL Draw Dark Charm
Plays it successfully defeats an hero
OL Draw Dark Charm
Plays it successfully defeats an another hero
etc

On your example I wold say yes he can again test poison !
As a hero can't have twice the same condition, by succeeding his first test, the condition his a new poison and so, from my point of view, he can decide to test it again .. he is always in it's start of turn and while he has action that he can do I don't see why he couldn't test again.

First off I must say.. wow, what an usual situation! I've played a a lot of Descent and yet have never seen this occur.. although admittedly my group doesn't explore Secret Rooms all that often.

I would agree generally with what Felin and you have already said.

If a hero is poisoned at the start of the turn, clears the poison, and then gets poisoned again during the start of the turn, I'd say they must test again for the new instance of poison right away.