This is part of (or at least adjacent to) that discussion from last week... It's entirely on the merchant to audit that they've paid a commission to multiple marketing channels for the same purchase, and some do while some don't. I don't deliberately double-dip (I'm getting less bold in my old age), but I've run into it with some merchants flagging a purchase in error when reconciling commissions and trying to claw the miles/rebates back on whichever channel was costliest or easiest to pull it from. Anecdotally, reading this thread though, it doesn't seem like a lot of merchants actually go through that much trouble, but in the cases where people report clawbacks weeks or months later, it's usually from a commission audit, and this is the type of thing they'd catch.

It's kind of a low risk thing though... Worst case, you'll still get the rebate from one of the portals, you just lose control over picking which one you get the benefit from -- at that point you get the one the merchant picks what works best for them.