Is your search returning more than 1 event when it runs ?
If it does, maybe massage your events, like using a "|dedup " or "| head 1" to trim them before the notables are created.
If your events results are unique but get indexed twice
check the _indextime of the notable events, to figure when they were created.
Check if you have a useack=true enable in the outputs.conf of your search-head (it can cause the forwarder to attempt to send the same events multiple time in case of network failure)
check if you are not cloning your data to 2 sets of indexers