When text fields has index_prefixes or index_phrases enabled, they create subfields of the form field._index_prefixes. These fields are only meant for internal optimizations and shouldn't be searched on directly.

However the text mapper exposes these hidden fields as if they were configured as multi-fields. This means that field caps includes the hidden fields, as when requesting all fields through fields=*. The search 'fields' option will also match against them, which can add duplication to the output.

Perhaps we should stop exposing these fields, keeping them an implementation detail. Note that the same consideration applies to search_as_you_type.