How do I find what is using a serial port (real or virtual)?

[Windows 11]

How do I find what is using a device? I am trying to use COM18 for a virtual com port, but I get an error "COM18 already exists". COM18 does not appear in Device Manager, even with Show Hidden Devices selected. Nor does it appear in a search of the registry. I'm running Windows 11. I would greatly appreciate any suggestions.

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