There’s a very simple reason for doubling consonants - it’s easy to understand and apply - super useful - and it’s not random at all!

I learned this in school, in England.

Think of the double consonant as ‘protecting’ the sound of the foregoing vowel.

If there was only one ‘s’ in ‘focused’, then the ‘u’ would really need to be pronounced ‘oo’ rather like the ‘oo’ in book (which is, by the way, another example of how double letters change pronounciation).

When you put the 2 ‘esses’ in - ie ‘focussed’ - the double ‘s’ ‘protects’ the ‘u’, and keeps it sounding soft, like the ‘u’ in ‘us’.

This applies to many words and is really why we have double consonants in English.

The double consonant is there to tell you how to pronounce the word.

Examples:

• Mating - Hard ‘a’ sounds like ‘mate’

• Matting - soft ‘a’ sounds like ‘mat’

• Rated - Hard ‘a’ sounds like ‘rate’

• Ratted - Soft ‘a’ sounds like ‘rat’

• mused - Hard ‘u’ sounds like ‘used’

• mussed - Soft ‘u’ sounds like ‘fuss’

It is perhaps the effect of the ‘following vowel’ that the double consonant is ‘protecting’ us from.

This is because of ‘the magic e’ Which transforns the vowel sound from soft to hard, as below:

Rat becomes the hard ay sound of ‘rate’ once it acquires the ‘e’. This is ‘the magic e’ Whereas an extra consonant ‘insulates’ the ‘a’ from the effect of the ‘e’ And so then, it still sounds with a soft ‘a’ as in ‘ratted’. The ‘e’ loses its effect - when the double consonant is there to ‘protect’ the first vowel - keeping it soft. It is no longer ‘hardened’ by the ‘e’.

This makes it easy to know how to pronounce even unfamiliar names, in English.

For example my name, Torrington. Can’t have a hard ‘o’ like toe’.

Because the double ‘r’ protects the first vowel, keeping it a soft ‘o’ like ‘pot’. It could only sound like ‘toe’ if it had only one ‘r’ eg ‘Torington’ So, as it has a double rr we can know, the first syllable must sound soft like ‘pot’.

In the examples above which appeared in an answer:

eavesdropped, kidnapped, formatted, worshipped, zigzagged etc.

They absolutely must have a double consonant, to sound the way they are supposed to sound!

For example - here is what would happen to their sound - if we swapped the double consonant for a single one:

• Eavesdroped - the o would sound like ‘oak’ and not ‘pop’
• Kidnaped - the a would sound like ‘nape’ and not ‘nap’
• Formated - the a would sound like ‘mate’ and not ‘mat’
• Worshiped - the i would sound like ‘wipe’ and not ‘hip’
• Zigzaged - the a would sound like ‘vane’ and not ‘gag’.

So none of them would sound the way they are supposed to.

And

Benefited - the i would sound like ‘bite’ and not ‘bit’. Benny fighted.

The double consonant is not a beast set there to confound you - it is simply a tool for keeping the integrity of the pronounciation of the word. And once you know that, and understand that it’s not random, at least some of English spelling will hopefully become easier for you - and you can know that;

• a vowel before a double consonant will be soft
• a word that has a soft-sounding vowel will need a double consonant after it, to keep it sounding that way.

Lastly - the word ‘focussed’ was traditionally spelled ‘focussed’ in the UK and ‘focused’ in the US, I think. But this has become a bit blurred.

Focuses - it would be pronounced foc-uses the ‘u’ sounding like ‘use’. Focusses - it would be pronounced ‘focusses’ like ‘plusses’.

However, most people do not seem to know the reason behind double consonants in English - and so some are probably making up their own spellings.