user: add multiple-inclusion protection

BUG=b:187790077
TEST=CQ passes

Change-Id: I2611464e98237acfd10bac39d885bcfa9a10d43f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/4111960
Reviewed-by: Lizzy Presland <zland@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed