X referred Z, a product. it did not work. Y referred same product after few months. Who gets commission

We run a product referral, and when two people refer but at different times, whom should we pay for the referral?

The first person might have referred but might not have done a good job of selling, or the need was not there then.

When the second person referred and converted, either they did a good job at selling or simply the need was high.