I encountered the "double decline" implementation of DCC in Portugal that others here have described with two point of sale terminals. In both cases the terminals were facing me. The receipt said Global Payments. I didn't see the markup on the first transaction, but the second one was showing as $4.05 for a €3.50 purchase, so 4.75%.

There was a food truck where I paid cash on a de minimis purchase since the merchant didn't appear to have a card terminal. The merchant did, but it was cashier controlled behind the counter. Sure enough, DCC was present. I know this because, in fairness, the cashier asked the customer after me whether he wanted £ or €. The guy correctly said €.