If you find a task with two people marked Accountable, that is not a quirk of this particular project, it is a sign the task itself needs to be split into two separate tasks, each with its own single Accountable owner.
The four roles are simple to state but easy to misapply, especially the Responsible and Accountable distinction. Get comfortable asking two questions for every task: who is actually doing this work, and who is answerable if it does not get done well. Those two questions, asked honestly, will get you most of the way to a correct RACI assignment every time.