A meta_query containing an `OR` relation can result in the same record matching
multiple clauses, leading to duplicate results. The previous prevention against
duplicates [18178] #17582 became unreliable in 4.1 when `WP_Meta_Query`
introduced support for nested clauses. The current changeset adds a new method
`WP_Meta_Query::has_or_relation()` for checking whether an `OR` relation
appears anywhere in the query, and uses the new method in `WP_User_Query` to
enforce distinct results as necessary.
Props maxxsnake.
Fixes#32592.
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