Amazon (at least in the UK) does have the "I already own this" option - along with an "I'm not interested in this" one. Some of their suggestions are a little

Amazon (at least in the UK) does have the "I already own this" option - along with an "I'm not interested in this" one. Some of their suggestions are a little odd at times - I've no idea how many different coffee machines they think I might want own - but they are mostly pretty good, and are probably the only place I've ever regularly bought recomendations from.