This doesn't seem clear cut to me. Sufficient and deficient have the same stress pattern, but deficient doesn't have a double letter. Neither does proficient.

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wdporter 7 hours ago | parent [-]

The ff in sufficient indicates a short vowel sound for the "u". The single f in deficient indicates a long vowel sound for the "e". Hence, suff-icient and dee-ficient. At least, that's how we pronounce it where I'm from.

	▲	atombender 6 hours ago \| parent [-]
		OED, Merriam-Webster, and Collins all show a short "e", as in "delete" or "different", i.e. /dɪ/. I've never heard it pronounced as /diː/. Do you have some kind of southern drawl where you put the stress on the first syllable, like some people do with "defense"?