Tosefta zu Terumot 4:14

A Samaritan that separated terumah and gave it to a [Samaritan] priest (see Lieberman), his terumah is [valid] terumah. Rabbi Shimon ben Elazar says, it is [valid] terumah but he goes back and takes terumah a second time [and gives to a Jewish priest, i.e., a Kohen]. They said to him, "What is the difference between this case and a Kohen that took terumah and brought it before his livestock [which is permitted]." He said to them, "The difference is this -- that this one separates terumah with sanctity, and that one does not separate terumah with sanctity."