What is the most likely cause if two users experience sporadic connections to a server while local testing shows no issues?

The most likely cause of sporadic connections to a server for two users, despite local testing showing no issues, is that two workstations have been configured with the same IP address. This situation results in an IP address conflict, where both machines attempt to use the same address on the network. When this happens, intermittent connectivity issues can arise, as the network may not be able to determine which machine should be responding to requests properly. This can lead to instances where one user's connection may work while the other’s fails, hence the sporadic nature of the connectivity issues.

In contrast, if two separate NICs had the same MAC address, this would typically manifest as a more severe issue, affecting all devices that attempt to communicate with either NIC. Multiple DNS entries for the same host might lead to confusion over which IP address to connect to, but it generally wouldn’t cause sporadic connectivity issues if the DNS resolution is functioning correctly. Incorrect routing for the clients could lead to complete inability to reach the server, rather than sporadic connections. Thus, none of these scenarios would logically lead to the symptom of intermittent issues as effectively as the IP address conflict would.