MySQL DISTINCT on a GROUP_CONCAT()

I am doing `SELECT GROUP_CONCAT(categories SEPARATOR ' ') FROM table`. Sample data below: categories ---------- test1 test2 test3 test4 test1 test3 test1 test3 However, I am getting `test1 test2 test3 test4 test1 test3` back and I would like to get `test1 test2 test3 test4` back. Any ideas? Many thanks!