BBC
Bbc: Skillswise: Suffix Spelling Rules: Double Letters [Pdf]
Learn when to double the final consonant of a word when adding a suffix, and when the consonant does not get doubled.
Grammarly
Grammarly Blog: I Before E Except After C: 9 Helpful Spelling Rules
This site provides nine important spelling rules and examples. L.9-10.2c Spelling, L.11-12.2 Cap/Punc/Spell