Cinefactus dixit:
If the vowel preceding the single consonant is long, is the one before the doubled consonant short?
Generally speaking, yes (when it comes to variants/by-forms/collateral forms, like Godmy said).
Plautus manuscripts prefer the long consonant form on the whole, and meter will indicate where compensatory lengthening has occurred (as in the case of ēs 'thou art' < Proto-Italic
*ess < PIE *(
H)essi).
Incidentally, it is guessed that the long consonant had a slightly deferential, or perhaps 'hypocoristic' (Sihler's word), connotation, specially seen in words like
flaccus, lippus, and
atta, and that this is the reason the form
Juppiter is universally preferred, even though comparative evidence supports
jū-. That is, you addressed the god in formulae as a child addressing its father.