Creating a custom filter without real field

I need to filter repeated fields (not nid, so distinct don't work) in
some views. I found how to make it using
hook_views_pre_render(&$view). I want to make a module to "tag"
displays that must be filtered.

To do so I have created a filter; where you choose which fields should
be unique. Then I am trying to use hook_pre_render() to eliminate the
results; but in $vews->result there are fields alias, and I just have
fields id. How can I translate field id to field alias?

Thanks.

(I can post code if needed)
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