2021-07-23 : Copies of Hellish Rebuke that weren't cast will give opponents permanents the ability, but that ability won't do anything because no player cast that copy of Hellish Rebuke.

2021-07-23 : If a player casts more than one Hellish Rebuke in a turn, permanents will gain multiple instances of the triggered ability. Each will trigger and resolve separately. For instance, if you cast Hellish Rebuke twice, each permanent which damaged you will cause its controller to lose 4 life.

2021-07-23 : If a spell or ability causes another player to gain control of Hellish Rebuke while it is on the stack, permanents controlled by the opponents of that player will gain the ability (probably including permanents you control). However, you're still the player that cast Hellish Rebuke, so the ability will trigger whenever one of those permanents deals damage to you, not the player who controlled Hellish Rebuke as it resolved.

2021-07-23 : If more than one player casts Hellish Rebuke in a turn, some permanents will gain multiple instances of the triggered ability that refer to different players. Each will trigger only for the player who cast the Hellish Rebuke that granted it the ability.

2021-07-23 : When the triggered ability that Hellish Rebuke grants permanents resolves, if that permanent's controller can't sacrifice it, they will still lose 2 life. Similarly, if they can't lose life, they will still sacrifice the permanent.