The instance.snapshot.start and .end notification sample file was
attached to two different Notification ovo by mistake. This caused that
the samples are appeared in the notification dev-ref twice.
This patch removes the duplication and adds a simple check to the doc
generation code to detech such problems in the future.
Change-Id: I8478c134f9cdb08c02a44e1838dbacede5523914
Closes-Bug: #1732685