@Ankur Bawiskar 
If we make field unique then race condition still trigger and script will create same unique id for both records, that will also cause issue i think.

What about before update BR because we have to create record from flow, if we update unique id field from before update BR, will it work? keeping in mind the same race condition for inserts from flow?

Or can we stop execution of flow instances or make it run once at a time?

Or can we stop querying the same record until first query is completed etc ??