Your screen shot is confusing. It looks like there are duplicates (the names with the "(1)" on them), but I don't see the actual original file names. Did you delete those, or did the ForumBackup addon?
Dropbox has an option on its account on whether it should overwrite files with the same names, but I don't see any files with the same names. But that doesn't appear to be the issue.
I would suggest that you change your date format to "d-m-y-siH" and turn on the addon's debug option, and send me the results. I'm curious also as to when you have your backups set to run via the cron tasks.
There might be a bug in the DropBox addon that doesn't recognize those duplicate names with "(1)" in them when it tries to determine the oldest files to delete. Technically, you wouldn't want to create 2 backup files with the same name, which is why I suggest you use the default datetime format of "d-m-y-siH".