Reintroduce DISTINCT keyword to BP_Core_User::get_users()

The lack of DISTINCT was causing duplicate entries when joining against the
xprofile fields for the purposes of alphabetical sorts.

Reverts part of r7700. See #5303

Fixes #5550

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8290 cdf35c40-ae34-48e0-9cc9-0c9da1808c22