That is odd. I can confirm that one gets the expected behaviour on Tcl/Tk 8.5.9 distributed with macOS and on the Tcl/Tk 8.6.11 available via Homebrew on macOS. The expected behaviour (as near as I can tell) being that both “one” and “two” are displayed when double-clicking because of reasons.
I can also confirm that one gets the unexpected behaviour on ActiveTcl 8.6.9 on macOS.
I suspect that this bug is related to earlier bug reported which deals of virtual events. Sadly I am not a C programmer and I am not willed to become one.
I have made some tools where the advantages of a great scripting language prove time advantage.