We only double the consonant in a word which ends with a single consonant (drop the "e") if it is following a syllable with a short vowel. If its a long vowel we keep a single consonant. (There is the odd exception I may have to look up) - how else would you know when to say "hopping" and when to say "hoping"?
hop - short "o", so double the p and add "ing"
hope - long "o", drop the e, keep the p singular and add "ing"
duck - already has two consonants, so just add "ing"
similarly,
work - already has two consonants, so just add "ing"
keep - long "ee", so just and "ing"
travel - short "e", so double the consonant and add "ing"
I suppose the US rule is to keep a single consonant unless there is a pre-exisiting word which has a single one. So as there isn't a word traveling it can be dropped (a bit tricky, I think).
