How large is your library? Sometimes for larger libraries with many edits the backups do get to be quite large. To answer your question though, yes the backups should be overwritten but I do wonder if you don’t have enough space available on Dropbox to even create and finish the second backup to it’s natural conclusion.
Have you by any chance looked into physical storage media for your Nucleus? We can look into the dropbox issue further but if you would like to have a much more stable backup experience I would always recommend adding physical storage backup, as this is not limited by Dropbox’s upload speeds or storage constraints. Plugging in a 64GB flash drive into one of the Nucleus’ USB ports will get you this ability.
Another method here would be to set up a NAS to talk to the Nucleus and have the backups saved on the NAS. Or if you don’t have a NAS but you are willing to dedicate some storage space from one of your PCs as a backup location, you can also configure a folder to be shared and accessible from the Nucleus by sharing the folder over the network. Please let me know if any of the above options would be of interest to you and we can help you set those up.