If you create a CallKit extension with Xcode 15 beta, then add some logging to the template code that gets created and run it on an iOS 17.3 beta phone the logging clearly shows that the OS is running the extension twice when the user turns on the extension in Settings. Run it on any device with < iOS 17 and its only invoked once, as it should be.
However, not only does the OS run it twice, but based on the logging the two invocations are in parallel, not serial, suggesting two CallDirectoryHandler instances are being created and run simultaneously.