Had the same problem, and someone pointed me to lemmy_migrate by wescode on github, and it worked well. Although it do require a recent version of python installed
So if multiple people with the same username from different instances comment in a post, would they differentiate from each other or will it look like the same person?
Depends on what one are using to browse lemmy with. I'm currently using voyager and it seems to show all as just their username, but for example now while replying to you, it shows that I'm replying up electrogamerman@lemmy.world, so I could use that to differentiate you from electrogamerman@lemmy.ml in that way. But no way to confirm if the lemmy.ml is just you on a different instance