https://bugs.openldap.org/show_bug.cgi?id=9634
--- Comment #3 from Michael Ströder michael@stroeder.com --- 1. Even when storing <what> clauses into a list you could maintain a separate bit field which types are already defined and reject multiple <what> clauses of same type during config parsing.
2. You could also simply treat <what> clauses of same type with and-relation. This could be useful in some cases. E.g. define a cheap <what> DN clause for skipping an irrelevant ACL more early in most cases but ensuring to only fire after applying all potentially expensive checks.