We do get the occasional transaction that doesn't have the correct reference (which enables us to automatically assign it to the right person). But this is usually just a technical error by the merchant relating to a transaction that is rightfully ours - and most of the time we can use other information to deduce which of our members it should be assigned to. I can't ever remember having a transaction that I had reason to believe should have been assigned to another site
The usual explanation is that, before using imutual's link, the member has used other websites to research their purchase and inadvertently triggered the other site's tracking cookie. Note that this doesn't necessarily involve clicking on that site's link; it could be requesting a quote through a comparison service or just having the merchant's banner displayed on the page they are viewing
Even though their subsequent click on imutual's link ought to override and previous cookies (known as the "last click wins" policy), this doesn't always happen; either due to a technical error or a deliberate decision by the merchant to give preference to the other website.