Salt grains.append with lists

[root@master ~]# salt testminion* grains.get roles
testminion:
[root@master ~]# salt testminion* grains.append roles foo
testminion:
    ----------
    roles:
        - foo
[root@master ~]# salt testminion* grains.append roles foo
testminion:
    The val foo was already in the list roles
[root@master ~]# salt testminion* grains.append roles "[foo]"
testminion:
    ----------
    roles:
        - foo
        - foo
[root@master ~]# salt testminion* grains.append roles "[foo]"
testminion:
    ----------
    roles:
        - foo
        - foo
        - foo
[root@master ~]#