Yes, no question that this possibility has added value and should be kept.
On the other side, how often is it really exploited in practice? 1% of the editing time? Less? Perhaps never for many users?
So most of the time it's probably rather an annoying usability issue, as one mostly ends with many duplicated open windows without a real need for them, which makes the workspace crowded and unclean.
I envisage something like that:
When opening a part (or a set of parts) for editing, jump to existing opened window with the same parts if there is one already. This should be the default.
If you really want to open a duplicate window explicitly, just hold Shift while double-clicking a part, or Shift-click the corresponding editor menu entry, or use a dedicated shortcut (e.g. Ctrl+Shift+E instead of the usual Ctrl+E for Pianoroll).
A slightly less elegant solution could be to simply switch the old/new behaviour globally by means of a user setting.
I have already implemented most of that in a branch, in fact. I was just not sure if I should go for the user setting option or the other one.