Double letters

Generally you need double letters to keep two short vowels away from each other. Short vowels are those that say their sound, not their name e.g. 'a' in 'hat' compared to 'a' in 'name'.

It can only apply to the stressed syllable - but this can be hard to hear.

Mostly, you do not need a double letter if there are already two consonants e.g. mother.

In week one spellings this rule is very helpful:

accommodate, accompany, according

You can see that accompany and according have adjacent consonants so do not need doubling

However: 'aggressive' does have a double 'g' even though there is an 'r' as well!