I don't want to offend by stating the obvious but, just in case something has been missed...
Any changes to a file in an import folder will cause it to be uploaded a further time. Is there anything going on on your computer which is causing the updated flag on the file to change?
Again, just in case, have you tried a complete and deep uninstall? It will mean reconnecting your import folders. It is possible that the glitch that was definitely around with 10.59 or thereabouts might have persisted.
On a Windows machine, uninstall using the free Revo Uninstaller then reinstall using the latest download. There's is a similar a program for Macs.
If you've tried these then my apologies for stating the obvious.