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create one block with some geometry inside and copy it (so there are 2 instances)
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open one of the instances and modify the geometry inside (close block edit)
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Ctrl+C one instance
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Ctrl+Z to undo block edit changes
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Ctrl+V to bring the modified block from the clipboard
(this will open the Block Name Conflict Dialog) -
Select “Both blocks” and click OK
So now, the modified block should be brought into the scene from the clipboard. It will have a new name like Block 01 01. Instead the unmodified one is pasted.
To see the correct behavior repeat those steps in Rhino5.