Now logging in using one of the imported accounts (that wasn't already in SD+) seems to work fine, but when logging in using an account that was already existing before merging with LDAP will sometimes (it seems pretty random) let me login using LDAP credentials. But it will always let me login using the Local authentication. 

I would have thought that when importing from LDAP it would overwrite the details of the local user, but it seems like there might be duplicates but they're not showing up. I've also tried deleting the user that was created before merging with LDAP, and re-importing, and it does the same thing.