This is a highly dubious practice, IMHO.
If these companies are associated, why can they not just move funds around via some inter-company transfer or a mechanism of their own choosing? Why do they need you to effect these cash-movements if its not questionable?
You are paying an invoice (presumably) inclusive of VAT, how do you know the 'associated company' is VAT-registered?
If the 'associated company' supplies the same products / services, why not order from them directly and keep things simple?
The minimum you will need is a proper credit-note from the original supplier and a fresh invoice from the 'new supplier', even then I'd be reluctant to facilitate them.