Pudge, the rule for doubling is simple: if the word is one syllable (got) or has the stress/accent on the last syllable (regret), and the last three letters are consonant-vowel-consonant (commit, but not comment; spin, but not rein), when you add -ing or-ed, double the final syllable (gotten/regretted, but spinning/reined). But, in the case of two consonants (comment), just add the endings (commented, commenting). More examples: spend/spending, but shed/shedding; bang/banged, but bag/bagged. You have made me thing this out and I can think of one exception: spray/sprayed/spraying.