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.
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.
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.