Doubling is triggered under what I call "the hamburger rule". If you have a closed final syllable - or one-syllable word (consonant-vowel-consonant), then you have a hamburger, which is made up of bread-meat-bread.
Well, almost. A ball of meat becomes a hamburger patty when it is squashed under stress. So it's really only a hamburger when the syllable is stressed.
And if it's a hamburger, then it has to be a double cheeseburger (Big Mac). So you double the last consonant before adding -ing/-ed.
The Hamburger theory TM
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Warning - this might make your students hungry!