Not sure if its been covered before but for information

I found I was getting loops in a imported schedule that I was updating in P6 with a schedule from a contractor.  I would only update the activities, relationships, and resources on the activities.   All the rest i would set to keep existing.   The contractor had made a number of logic changes in the plan.   I imported and went to F9 the project after some links to outside milestones and I suddenly had loops appear.   I was thinking that they were in the contractors XER they had submitted.   I tested thiers as a new project and F9 worked fine.

I removed the loops and imported the plan again and got the loops again.    Finally I stopped to read the delete check box in the import modifcation area and I deleted all the relationships in the current plan and replaced the same with the import plan.  I didnt have any more loops 

Just thought maybe you could use this info for those ever running into the same problem.

I think the import plan was simply adding the logic which was new to the logic already there.