The cause of English consonant doubling errors is NOT insufficient teaching of grammar rules, but the chaotic nature of the doubling system. It IS REGULAR ONLY, when one-syllable words, which have a short vowel and end in a consonant, are made longer with suffixes like -ed or -ing (fitted, fitting). This is to differentiate them from words with long vowels (sited, siting).  With longer lengthened words, doubling is also meant to occur after short vowels only (permitted - vomited), but in the UK esp. this is erratic (travelled).  

In longer root words, doubling is totally unpredictable:
418 wds double, e.g  'rabbit, poppy', but
554 don't (habit, copy),  and  218 double for reason of Latin grammar, (abbreviate, apply),
rather than for keeping a stressed, short vowel short.
I have explained the problems of the long/short vowel system in more detail on my ImprovingEnglishSpelling blog.  

Most spelling mistakes of Anglophone adults are due to the randomness of English spelling.