As I am sure you have gathered you did not need step 3 where you did the import of the data. Joplin treats the import as new notes so when the client was then synced as well you ended up with two of everything: one copy from the backup and another from Dropbox.
Personally I would clear out everything and start again.
The linked post below has a method for doing that. However in your case you have already completed step one. Do not do another JEX backup as that backup will contain two of everything!!
The method is a bit "wordy" just to make things clear. It is not actually quite as complicated as it may look.