Yep, there used to be tons and tons of dual-branded hotels in The Luxury Collection. That chain began in 1992 as the ITT Sheraton Luxury Collection, of course, so almost all the hotels in it were originally Sheraton-branded. When ITT Sheraton bought CIGA in 1995, they put almost all of those new hotels in the ITT Sheraton Luxury Collection, like The Palace in Madrid, but didn't give them the Sheraton name. When Starwood bought Sheraton in 1998, they rebranded a number of those former CIGA hotels as Westins, but left them in The Luxury Collection for a transitional period of a couple of years, before they were fully moved out of The Luxury Collection.

As for all the Sheratons in The Luxury Collection, later owners of the chain have clearly pushed to get rid of them, to make the branding clearer, but they're all clearly under long-term contracts, so it's taken many decades. When Starwood took over, there were dozens of Sheratons in The Luxury Collection. Today, only 3 remain - Bangkok, Kuwait and Addis Ababa, and you can bet Marriott is eager to see those branding contracts end, so they can be renamed or moved to the Sheraton brand.