In my previous posting, I discussed the contrast between wholly (with a double /l/) and holy (with a single /l/), and I suggested that in English this contrast might only occur in medial position with /l/.
A correspondent, Peter Tinkler, has suggested that thinness and thinnest may exhibit a similar contrast, with thinness having a double /n/ (because the suffix is ‑ness) while thinnest has a single /n/ (as the suffix is ‑est). Given that the final /t/ in thinnest may sometimes be omitted because of consonant cluster reduction, these two words may indeed potentially be distinguished solely by means of the length of the medial /n/.
This is pretty convincing to me; so it seems that not only /l/ can be geminate in English.
Other examples of doubled medial consonants in English are bookcase and part‑time, with a geminate medial /k/ and /t/ respectively. But note that both bookcase and part‑time are compounds, while thinness and thinnest involve suffixes; and I think the latter examples work better.