One obvious choice is that there's only one maintainer doing merges -- if they sign and push different content to different repos, it's a problem of their own making. So unless they screw up, there's no need to worry.

Other than that, you could give priority to any change based on e.g. its timestamp or hash. Sounds arbitrary? Yes, but such is life. If two people push a branch to be merged simultaneously to a repo where everything is merged to master after rebasing, one of them (depending on luck) is going to find that their commits landed while the other soons finds out that they need to rebase and try again. In a scenario with multiple mirrors, you'd obviously want to try to use the same repo when possible, but if you don't, it'll take a little longer for you to find out that your stuff was overruled by changes in another mirror.