I'm seeing two issues I feel are related that are hindering my Ancestry Sync (that I still have months of work to do to complete).
First off I'm finding a large number of duplicated marriage events. Often this will be the same couple, but reversed. For example John Smith and Jane Doe will be married once with John Smith as father and Jane Doe as mother, another with Jane Doe as father and John Smith as mother. The only way I can get rid of these is to manually remove them in Ancestry, and these events only exist on the Ancestry side.
I'm see also what I believe to be a symptom of the above issue, significant duplication of the children of the duplicated marriage. If were are 6 children, there will be 12, with 6 being half-siblings with Jane Doe as the father and John Smith as the mother. The only way to fix this is merge each in Ancestry, and then manually delete the reversed parent relationship. These children are identical duplicates of their half-siblings, and they only exist on the Ancestry side.
The shear volume of these (I'm dealing with many hundreds) shows the only way this could be happening is via automation. I can't tell if the issue is from the RM side or the Ancestry side, but I do know the only way to fix it is on the Ancestry side.