We have the ability to enforce unique email address from self reg. We should have a staff side equivalent. When using SSO it can be problematic to have the same email address on different accounts.
For long time many wished to have a unique email in Koha that could serve as userid too. This self registration pref is just a kind of partial implementation of that. The response has always been: But we need to have duplicates for parent and child accounts. We ended up with the current code ;) I am trying to get this moving again on bug 32426 now. No trivial operation sofar.