You know they are the same user because mastodon has a protocol for Migration. Basically, you announce on the old instance what your new handle is and confirm from the new instance that this is where you came from. Mastodon then migrates your followers (and blockers). So you can change instances transparent to others.
Of course, this migration is more work than simply creating a fresh account, so i doubt this helps against spammers, but it may help against annoying people.
testing