When an unhappy couple’s argument causes the man to storm out, he orders a service to resolve the fight on his behalf, unaware that a doppelganger is stealing his relationship in the process.

A couple arguing could imply that they're unhappy, rather than stating it twice. The first half is from the couples POV and the second half is from the man's POV. Considering picking one POV to make it read more consistently. Generally 1 POV is ideal for a logline. Great twist! Consider looking at how to structure a longer logline with a mid point reversal (in the formula section of the website) in order to make it read more cleanly and potentially explore his action after he discovers the doppelganger is stealing his relationship :)