If a duplicate file has been created, the Writer app can't reliably determine which file it's synced to in your Dropbox folder.
The remedy is to rename your local copy of the script to some other file name, such as "filename today's date.
An alternative to this method is to access your Dropbox account via Dropbox.com and locate the file that ends with "(conflicted copy)". Rename this copy of the script to some other file name, such as "filename OLD."' Be sure to remove "(conflicted copy)" from the file name.
Renaming one or the other of the files should prevent the warning from coming back. Once this is done, you can sync the file to Dropbox from the Writer app again.