Ensure `get_terms()` results are unique when using 'meta_query'.

The introduction of 'meta_query' to `get_terms()` in 4.4 made it possible for
`get_terms()` to erroneously return duplicate results. To address the issue,
we add the `DISTINCT` keyword to the SQL query when a 'meta_query' parameter
has been provided.

Props @jadpm.
Fixes #35137.
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