Looking to help someone in need (not me!) – just need to know the issues if I do this on scale

So QBO has, approx. 18 months of accounts. I have been ‘helping’ with their books over the last 12 months and it’s spot on with matching up with the bank feed.

Unfortunately the previous period from initial set up to my help has loads of double entries and so the balance between QBO and the bank is way out.

Can I basically just go in and delete those duplicates? i.e. removing the double spends etc so that QBO matches correctly to what has actually been transacted in the bank during that time period?

I know from my own QBO accounts that any change made to data in a previous VAT period gets ‘corrected’ on the current/next return.

Hope my query makes sense…..

Cheers muchly