Say if I were to cast Dominate Beast on a beast that already has telepathic connections or is telepathically controlled by an enemy...  will the enemy controlling it have to succeed on a wisdom saving roll as well in order to maintain control of the beast/beat out my attempt? I'm curious, as I suspect my group and I will be playing Ghosts of Saltmarsh, and I'm curious as to whether or not a druid or a sorcerer could use the Sahuagin's sharks against them.

If the beast is magically controlled, it's mostly up to the DM which spell takes effect.  If the beast is controlled by communicating with it and convincing/training it to obey orders, the magical domination will override that.

However, that's largely irrelevant since the creatures in question are not beasts but monstrosities, requiring Dominate Monster instead.

Quote from Lunali >>

If the beast is magically controlled, it's mostly up to the DM which spell takes effect.  If the beast is controlled by communicating with it and convincing/training it to obey orders, the magical domination will override that.

However, that's largely irrelevant since the creatures in question are not beasts but monstrosities, requiring Dominate Monster instead.

 
They're referring to the sharks not the Sahuagin so it would be Dominate Beast, although Dominate Monster would work as well if ya want to use a high level slot for a lower level effect.

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Anyway, Sahuagin do not command sharks by magic spells, they just use an ability, and I say the spell would override that. However, this is really a DM call, so best to talk to your DM.

Unless there's a shark from the MM that gets referenced that I didn't notice, the sharks in question are Shell Sharks making them monstrosities, not beasts.

Shell sharks only appear at Chapter 7 of the adventure, they face Sahuagin and sharks much earlier in Chapter 1. The original poster mentioned they have not yet started this adventure.

So what I said still stands.