There seems to be only one buffer for both actions, copy+paste and duplication.
Not sure how other applications handle this, I would have thought they are separate things, meaning i can copy A, then duplicate B, then paste A. In Pd the last step will paste B.
in Pd, duplicate is really just a shortcut for copy+paste. you're right that other applications handle this differently (e.g. REAPER) but for Pd I don't know if it's worth the hassle to add dedicated undo/redo actions for duplication...
I do think these usability things are important, but maybe that's something to throw out with tcl/tk alltogether.
it has nothing to do with tcl/tk, though
esp. all the copy/paste & duplicate logic is implemented in the Pd-core.
(that is: tcl/tk doesn't even know about any such thing)
Then this issue shall persist until it is solved.
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it has nothing to do with tcl/tk, though