Duplicate taxonomy filter in multi post type search

I use **multi-post-type** branch.

Let's say I register a new taxonomy called **genre** for post and page types.
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When searching for a post, the **genre** taxonomy filter is shown twice.

I'd expect the filter to be shown only once as it's common for both post types.

Reproduce the duplicate genre filter on the multi-post-type branch using a taxonomy registered for both post and page types. Trace where the search filters are assembled and identify how shared taxonomies are added; done when the common taxonomy appears only once.

Written by the indexing model from the issue text.