Yes, it was previously requested but that request got closed without a single reply so I'm requesting it again.
The duplicate note function is very useful although it might be improved. I've found that when I duplicate a note I generally (perhaps 99.99999% of the time) want to edit the newly created note. Unfortunately, the original note is the one with focus and I often (perhaps 50% of the time) modify the original instead of the new note. This tends to screw up the last modified date for that note not to mention leaving a note with "- Copy" tacked on the end.
A competitor of Joplin's does things a bit differently. When a note gets duplicated the focus goes to the new note and the title is automatically selected so the user needs only to hit the "Duplicate" (or whatever the equivalent is) button/menu and start typing.
It would be great if Joplin could at least set the focus to the new note rather than leaving it on the original.