I just did some experiments, and I think this should help identify such merge commits from the commit IDs of two copies:

Run this in bash (if you're using Windows, use "Git Bash"). Make sure you replace the `<hash>` tokens with the appropriate commit IDs. The hash it prints should be the merge commit tying the two copies together.

  commit_1=<hash>
  commit_2=<hash>
  
  awk '
          ARGIND == 1 { h[$1]++ }
          ARGIND == 2 && h[$1] { last = $1 }
          END { print last }
      ' \
      <(git log --ancestry-path --format=%H $commit_1..HEAD) \
      <(git log --ancestry-path --format=%H $commit_2..HEAD)