Last session ran into a scenario where dominate person conflicted with a suggestion. I wonder if I was going to rule it fairly. See below.
BBEG cast dominate person on a PC who carried a weapon artifact that was a piece of the BBEG. If the BBEG could reunite with the weapon, it would become very powerful. Otherwise, it was a potent weapon to be used against the BBEG.
BBEG mentally tells the PC to "bring me your weapon." The PC casters recognize the spell (Arcana check), realize what's probably happening, and the Bard successfully casts a suggestion on the victim PC that the PC "give your friend bard the weapon and use the weapon you've been using your whole life (her beloved intelligent greatsword)."
We had two conflicting spells with two conflicting interests. My initial ruling was going to be that the higher level spell wins. However, I never got there because the party's wizard intentionally tagged the PC with some damaging spells so she could get another saving throw on dominate person. She succeeded, so I didn't have to adjudicate which one prevailed.
So one thing to consider, technically the suggestion is not even needed in this case, at least initially. The PC could have given the caster any weapon on his person, which would be in line with the command given. Only if a follow up command “give me that specific weapon” would the person have to give them the artifact.
Basically unless the caster takes total control, the PC can interpret the command to minimize issues when possible
I don't understand the folks saying dominate person somehow erases suggestion. Are you really arguing that any mind-affecting spell can be used like dispel magic to wipe away lower-level mind-affecting spells?
Yes. Just rule that the higher-level spell takes precedence. Nice and simple.
Particularly with Dominate, as it gives total control over the target; taking away the control given by Suggestion, Charm, or the target's own free will.
The argument would be considerably fuzzier were the question one of, say, Suggestion vs Charm, as neither give total control over the target and the target's own free will can still get involved. Those can get messy, and have IME.
If the spell is the same, or the same level, I'd agree; e.g. a target under two Suggestions at once (though ideally the Suggestion spell wording would cover this by stating a newer casting overwrites a previous one, in order to avoid just this sort of mess).
Charm can be the really messy one, particularly when several people have Charm as a spell or ability and all try to use it on each other.