I recently installed the dropbox desktop app on my macbook. today I started my mac and dropbox asked me to "sign in" again, so I did. But the next steps suggested that I already had a dropbox folder on my mac and if I wanted to rename the new dropbox folder. So now I have two folders from the same account with the same content. One is already synced and has the name "dropbox old" and the other one is currently syncing the very same content. How could this happen and how do I get back to having just one dropbix folder.
Oh, and I have set the smart sync setting to "only online" sync.
Since you were asked to sign in again, it sounds like your computer was unlinked from your Dropbox account.
In this case, I'd recommend letting your desktop app sync your content for now. Once it's complete, you can check if all your important files are in the new Dropbox folder and are up to date, before deleting the previous/old Dropbox folder.