Today I realized belatedly that _philter_ (sometimes spelled _filtre_ in older English) is derived from the root of Greek φιλεῖν “to love”, and has nothing to do with English _filter_ (from PIE *pelt- “strike”—which yields _anvil_, _felt_, _pelt_, _pulse_, the _-peal_ in _appeal_ and similar words, etc).
So you can filter a philter without committing a redundancy. In case you were worried about that.
