What happens when an infernal summoner who has pact with a ghost to serve them encounters an Hermetic Magi casting Coerce The Spirits Of The Night? Who's commands will the spirit follow? How would you resolve this conflict?
Very brief: spells don't have resistance. The last successful spell controls the spirit.
OneShot is correct. An example of this principle is made explicit in "Command the living fire" (RoP:M 133).
I would resolve by having each contesting magician have to wrest control via a series of spells.
Summoner has cast a spell giving him magical control of a ghost. He sends it to attack Magus.
Magus casts a spell to control the ghost. This works, and Magus now has control of the ghost. Magus sends the ghost to attack Summoner.
This could volley back and forth for a while. If they are not in close proximity, quite a while.
If they are in close proximity, though, there's a order of operations issue and the two magicians might shout out threats, promises, and other persuasions. This becomes more a role-play issue. Pacts require negotiation. Coerce the Spirits of the Night involves interaction. The ghost might be self-willed enough to prefer to obey one magician over another, lending a bonus to one effort over another.