[BUG] ACL should not allow duplicate ACE entries in sequence

[adamretter]

At the moment it seems it is possible to add the same ACE to the ACL in the same sequence.
It is possible to create the following ACL at the moment:

Because these entries are in sequence and are identical, then should only appear once. That is to say that when adding a new entry to the ACL, if it is the same as the previous entry, then doing so should be a no-op that returns a success result.

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We likely need to add a check in the functions that implement ACLPermission#addACE(...). See:

1. exist-core/src/main/java/org/exist/security/ACLPermission.java
2. exist-core/src/main/java/org/exist/security/SimpleACLPermission.java
3. exist-core/src/main/java/org/exist/security/internal/aider/SimpleACLPermissionAider.java