I have two Glasc Mixologists in my Battlefield, my opponent clears both in by combat, I put both Mixologists on the chain, the first one to resolve reborns a Karthus. Does the Mixologist which is resolving second trigger twice?

No, the second Deathknell trigger will not trigger twice.

According to RiftJudge FAQ #9606, the number of times a Deathknell ability triggers is determined at the moment of death, based on the board state at that exact time. Because Karthus, Eternal was not on the battlefield when the two Glasc Mixologist units died, he was not present to modify those triggers.

Sequence of Events:

1. Simultaneous Death: Two Glasc Mixologist units die while Karthus, Eternal is not on the battlefield.
2. Trigger Addition: Each Glasc Mixologist triggers once (its base Deathknell trigger). Because Karthus, Eternal is not present, no additional triggers are generated (Rule 808.1.d.2).
3. Chain State: Two separate Deathknell triggers are added to the chain as pending items. The total number of resolutions is now locked.
4. Resolution 1: You resolve the first Deathknell trigger and play Karthus, Eternal from your trash to the battlefield. He enters play.
5. Resolution 2: You resolve the second Deathknell trigger. Although Karthus, Eternal is now on the board, he is too late to modify the trigger, as the number of resolutions was already locked in when the units died and the trigger was initially placed on the chain.

In summary, the presence of Karthus, Eternal only modifies Deathknell triggers that occur while he is already on the battlefield. He cannot retroactively increase the number of triggers for Deathknell abilities that were already placed on the chain before he entered play.