Session setup fails due to session hash collision error
This behavior can manifest itself in unique ways depending on the implementation. For this scenario, a user may reconfigure their source NAT from Dynamic IP and Port to Static NAT. This will allow the second call to match the initial s2c flow instead of creating a new session by the firewall.
When session_hash_insert_duplicate is seen in the global counters, keep in mind how NAT policies could affect new sessions that match exisiting flows.