We were one of those "people/couples" Carol. Years ago at Bliss when they had dining we didn't realize a service charge had been added so we tipped on top of that. We were staying at the Caravanserai so we did not drive. Too much sun, too many cocktails and well....
When we figured it out later the server or restaurant manager because who knows who gets the "service charge/tip" winded up with a 35% tip.
If there is 15% added we typically do not leave additional monies. There are of course exceptions.