Focused is the most common and widely accepted spelling of this adjective. Focussed is an old spelling that is still sometimes accepted but much less common.
The confusion about the spelling of focused comes because you need to double the consonant when you have a single letter stressed vowel preceding it.
You do not need to double the consonant in focused because the stress is on the first “o”.
The most common spelling in American and British English is focused.
Focussed in Australia, New Zealand, UK, Canada, India
From my research Focussed is not common in any of these countries although you might find it in some newspapers like the Sun.