Thanks for your response, Jeff. I experienced this issue in a previous library (and, if I recall correctly, in both 10.1 and 10.2), so perhaps it’s some preference that’s set funny. FCPX preferences are laughably pared down, as I’m sure you know. It seems to be a legit FCPX operation, but one I just don’t understand. Perhaps the developers assumed that those using the program would never have reason to duplicate sequences, and so this automatic opening behavior is supposed to be a convenience? The going wisdom among film editors seems to me to be that FCPX has lots of nice features, but is ultimately a program designed for amateurs working on much smaller scale projects than entire features, and that Apple is only gradually catching up.
At any rate, I guess I can try trashing my preferences and see if that clears things up. As I said, it’s not insurmountable, just irritating.