I have the default message/preview pane layout. Double-clicking a new messages doesn’t always open it in a new window. It seems like you’re trying to preview a message in the preview pane prematurely by not correctly determining if a click event is single vs. double.
This problem seems to occur mostly on the first time you try to double-click to open a message; it seems that subsequent times work OK.
Not a big deal, but a definite annoyance.