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.
This is a rather separate issue than the similar bug #32426, and bug #29480. I was pointed to this ticket from ByWater Solutions Support upon finding out the hard way that SSO does not function when there are multiple instances of the same e-mail address. I had been testing some things and entered my user's e-mail address into the primary e-mail address field of a test user. This was all fine, until I needed to re-authenticate (the next day). Upon which it sent me into an authentication loop. Removing my e-mail address from the primary e-mail address field of the test user fixed the problem. It would be great, if the Primary E-mail address was a unique field.