First, to clarify: multiple merges are never completed at the same time. What we're talking about here is permission being given from one side for multiple merges to be completed. Permission still needs to be given from the other side, and each merge still needs to be done by a member. (The permission and merge from the other side generally happen in one step, but still, each merge needs to be done by a WikiTreer, hopefully with appropriate care and consideration.)
Second, it's been mentioned that permission is being given to merge profiles for which the person is not a profile manager. Linda wrote: "If the profiles being proposed for a merge have the same PMs as all the ancestors 5 generations back, that would be fine, but normally they don't. Therefore when a merge is proposed for profile A and B, a PM of both A and B approve the merge, they can complete that merge, BUT that approval is also allowing any merge of A and B's ancestors to have merges proposed and they are 'automatically' able to be completed because the PMs of A and B said that 'one' merge was OK. ... Therefore the PMs of the 5 generations back have 'no say' in the approval process of their profiles since they were never notified of a proposed merge for the profile that they are PM of."
If this is happening, it's a bug and we need to fix it. (Jamie, can you test to confirm this? These are what we refer to as "implicit merge proposals". See BT #416 etc.)
Leaving aside the possible bug, do we want to make a policy change? It would be a significant one. The current system may seem a little wonky, but it's been in place for over 10 years. The consequences of changing it, I think, would be much more troublesome than most members would imagine. There would be far more merge proposals getting emailed back and forth, far more delays for permissions, and all accompanying frustrations. And, of course, more duplicates that never get merged. Making the merge process slower and stickier may cause more problems than it would solve.
P.S. I do think Ellen's idea is worth consideration, i.e. a technical change that would show all the potential merges for which permission is being given. If the user interface was done well that could be great.