<oo-admin-check-sources> Bug 1028572 - uniquify flatten results
https://bugzilla.redhat.com/show_bug.cgi?id=1028572

This bug was caused by `flatten()` allowing duplicate values in its
results. Wrapped returned list in `list(set())` to guarantee
uniqueness
Merge pull request #170 from dobbymoodge/BZ1028572

<oo-admin-check-sources> Bug 1028572 - uniquify flatten results