English Translation
Everything at that store is 10 percent off the regular price.
Natural: すべての商品を10%値引きしている。= Discount 10% from all products
Unnatural: すべての商品を10%値引きをしている。= Execute a 10% discount from all products – “Execute” sounds awful in English so as 値引きをする does in this Japanese sentence structure. Particularly, reduplication of を in the 商品を～値引きを phrase sounds unnatural.