You can use focused or focussed
She focused/focussed her attention on the details.
Do you use focused or focussed?
Both focused and focussed are acceptable.
Similarly, you can use focusing or focussing.
My preference is focused because I think it looks cleaner.
Did you know that we tend to double the consonant with verbs of two syllables when the stress is on the second syllable, e.g. rejig becomes rejigged?
But this rule does not apply to focus because the stress is on the first syllable.
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Read more about when to double consonants.