I recently noticed a pattern in loans from Hebrew into Greek: the letter šin (or sin, or łin if you're really archaic) is transcribed σσ after a short vowel, σ elsewhere.
My knowledge of Classical Hebrew and Aramaic is weak, so this gemination might be a feature of those languages for all I know. But off the top of my head, I also can't think of any Greek word with a doubled consonant after a long vowel.
Does any such word exist? Metrical evidence won't do us any good here, since the doubled consonant would make the syllable long by position—but η and ω have their length shown clearly in writing.