1. Create a simple route called "red". One way point will do.
2. Add a flight called "foo".
3. Duplicate the route "red" and rename it "blue". Note there is not flight showing on the duplicate.
4. Add flight the the route "blue" and call it "bar".
5. Save this mission.
6. Close the editor and then reopen and load the mission saved in step 5.
7. Note route "blue" now shows flights "foo" and "bar".
Not sure if the bug is that it copied the old flight and shouldn't have, or if it should have and it was just a bug that it wasn't showing it.
I could see both being reasonable, but if I had to choose I'd say a user would expect everything copy over with the route, segments and flights.
If that is the desired intent, then I guess the bug is that it didn't initially show the copied flight in the tree after duplicate.
