Initially, my question was: is "focussed" or "focused" the correct past tense of "focus", but since this applies to a lot of words, I would like to generalize and ask: is there supposed to be a rule when to double the consonant?
The rules are much more complicated, and I don't think it's a good idea to post them all here.
Re: doubling of the final consonant in an unstressed syllable.
Pam Peters (in "The Cambridge Guide to English Usage") argues that when the final syllable is identical with a monosyllabic word, the final consonant is also doubled in British English:
eavesdropped, kidnapped, formatted, worhsipped, zigzagged etc.
Michael Swan argues that doubling in such cases is caused by a full vowel, which hasn't been reduced to a schwa.
Burchfield, the editor of the most current Fowler's, also mentions such words, as benefitted, targetted etc., without any explanation. (BrE) It's interesting that Fowler's recommends "benefitted", whereas Garner's recommends "benefited" and argues that "benefitted" is wrong ("commonly misspelled").
Final -m is usually doubled in BrE (programmed); final -l is often doubled in BrE (cancelled) etc.
The most common variant is "focused" and "focusing", both in BrE and AmE (The Longman Grammar of Spoken and Written English).
The rules are more standardized in AmE (canceled, sometimes even programed etc.)
A note on "programed": I don't use this form. It is non-existent in BrE. It's listed in all major American dictionaries as acceptable.
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Both spellings are used depending on the variety of English. According to Wiktionary:
According to the website of a UK-based company Future Perfect, the general rule is as follows:
(Here, if the preceding vowel is the only vowel in the word, it is counted as stressed.)
However, I do not know how the spelling “focussed” fits this rule. Maybe it is an exception to the rule.
I believe the rule is that you double the final consonant when both of the following are true:
As 'cus' is not the stressed syllable, it would not be doubled according to this rule.
I believe the 'stressed' requirement is relaxed in some situations under British English, however. Indeed, I found references to 'focussed' with an 'especially British' tag.