OPC clients do not directly "start" OPC servers, and have no influence on how they are started. OPC clients just connect to servers. The "connection" may, in some cases, involve starting of the server process. When and how the OPC server is launched is determined and performed by the COM/DCOM infrastructure in Windows, and can be done in many ways. For example, an OPC server may be pre-started as a Windows service. Or a Windows service may be started when the first client attempts to connect. Or, when the OPC server is not a Windows service, COM/DCOM will also attempt to start it upon first connection, when it is not running.
Some (or most) servers expect that they are started only "once", i.e. that their process is started simultaneously two or more times. The error message you are seeing probably comes from the OPC server itself, when it has detected that something has attempted to start it for a second time.
Normally COM/DCOM will only start one instance of the server process, BUT it can also be configured in such a way that multiple process instances can be started. For OPC purposes, this is mostly incorrect (but can be correct for other purposes).
One can often see multiple instances of OPC server being started when in DCOMCNFG, the server is set to run under "Launching User" or "Interactive User" identity, instead of a pre-configured, fixed account. For example, when the server identity is set to "Launching user", COM will start a separate server process for each different OPC user accessing the server.
I suspect that this is your case. Do you have more OPC clients connecting to the same server? And, what are the user accounts under which the CLIENTS are run? What is the setting on the "Identity" tab of the OPC server app in DCOMCNFG?
Best regards
