Remove needless repetition of words from a text including repeated words with apostrophes

This expression, run interactively within Vim:

:%s/ \v(\w+('\w+)*)\_s\1/ \1/gi 

It removes needless repetition of words.

In this way it turns this:

the the it it them them it's it's then then and this too 

into this:

the it them it's then and this too 

as intended.

However the same expression used within vimscript, like this,

silent! exe ':%s/ \v(\w+('\w+)*)\_s\1/ \1/gi' 

fails to replace any repeated words.

I have tried:

• escaping the ' with a preceeding '
• escaping the ' with a preceeding \
• enclosing the expression in double quotes ":%s .... " and not escaping the '

but none of these worked.

I also tried:

silent! exe ':%s/ \v(\w+('\''\w+)*)\_s\1/ \1/gi' 

This did not work either.