That's not really a wise thing to do in my opinion. C'thuga is a GFE so you can sell it for 15k MP. A Tamadra costs 3k MP, and next event you're going to get 10 of them in your mail. And nowadays 50 plusses equal less than 50 stam (someone can carry you in coop in the dungeons where the boss drops as +30). 15k MP has more value than that. But even then, you shouldn't sell it just like that. If you lack space, spend one stone to expand your box and keep it. You should only sell him if you need the 15K MP to buy something in particular.

Keep the other 2 dupes because you never know when Gungho gives it an equip evo or something that makes dupes relevant.

Also, if you wait long enough (cough) there's an update where you'll be able to sell him and keep the plusses, so that's a win-win if you can wait until then.