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!