No. Replacement effects only get one chance to be applied to an event or modified event. So if noncreature source X would deal damage to a player or planeswalker, instead X does not deal damage and Comeuppance deals damage to the source's controller, then the other Comeuppance makes the first one deal no damage and deals damage to the player that X would have damaged in the first place (or the controller of the planeswalker repectively). That player's Comeuppance cannot be applied anymore, because it already was. Thus in your scenario in the end, whichever player has a noncreature source deal damage to the other player or his/her planeswalker will instead get to deal that much damage to the other player.
(If X is a creature, the other player's Comeuppance doesn't apply as it only prevents damage to the player and his/her planeswalkers, not creatures.)
Isn't Comeuppance a prevention effect, not a replacement effect?
You are correct. Comeuppance is governed by 615.5.
615.5. Some prevention effects also include an additional effect, which may refer to the amount of
damage that was prevented. The prevention takes place at the time the original event would have
happened; the rest of the effect takes place immediately afterward.
But see 614.5, which Rezzahan refers to:
614.5. A replacement effect doesn’t invoke itself repeatedly; it gets only one opportunity to affect an
event or any modified events that may replace it.
Chapter 615 (Prevention Effects) lacks a comparable rule. A 615.5a should probably be added that explicitly states that a prevention effect's additional effect cannot cause that prevention effect to invoke itself repeatedly, mirroring 614.5.
Since the effect of Comeuppance lasts till end of turn, and if both players play it, I presume damage that come later would again trigger two replacement effect, with neither side taking damage?
Rezzahan is incorrect. What we end up with if there are two Comeuppance in effect from different players that prevent and deal damage back and forth, is that the game gets stuck in a loop and ends in a forced draw. That's indeed because these are prevention effects and not replacement effects, and also as shown in 615.5, the damage dealt by Comeuppance following it preventing damage is a separate event. See this judge blog post: https://blogs.magicjudges.org/rulestips/2014/12/comeuppance-and-blocking-creatures/
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No. Replacement effects only get one chance to be applied to an event or modified event. So if noncreature source X would deal damage to a player or planeswalker, instead X does not deal damage and Comeuppance deals damage to the source's controller, then the other Comeuppance makes the first one deal no damage and deals damage to the player that X would have damaged in the first place (or the controller of the planeswalker repectively). That player's Comeuppance cannot be applied anymore, because it already was. Thus in your scenario in the end, whichever player has a noncreature source deal damage to the other player or his/her planeswalker will instead get to deal that much damage to the other player.
(If X is a creature, the other player's Comeuppance doesn't apply as it only prevents damage to the player and his/her planeswalkers, not creatures.)
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You are correct. Comeuppance is governed by 615.5.
But see 614.5, which Rezzahan refers to:
Chapter 615 (Prevention Effects) lacks a comparable rule. A 615.5a should probably be added that explicitly states that a prevention effect's additional effect cannot cause that prevention effect to invoke itself repeatedly, mirroring 614.5.
Shu Yun, the Silent Tempest WUR Voltron Control
Temmet, Vizier of Naktamun WU Unblockable Mirror Trickery
Ra's al Ghul (Sidar Kondo) and Face-Down Ninjas
Brudiclad, Token Engineer
Vaevictis (VV2) the Dire Lantern
Rona, Disciple of Gix
Tiana the Auror
Hallar
Ulrich the Politician
Zur the Rebel
Scorpion, Locust, Scarab, Egyptian Gods
O-Kagachi, Mathas, Mairsil
"Non-Tribal" Tribal Generals, Eggs