Backup ends with a single file if files have the same name

[peerdavid]

As reported by "silenzi71" in issue 17:

"If there are more documents with the same title (as it may happen with sharing files from apps or websites to the tablet), the result is an overwriting in the backup, with no message or control on that behaviour. So, instead of having multiple documents with the same "title", like happens on the tablet, you end with a single file (which will be only one of them, you will never know which). In this case, a numbering routine might solve the problem and create, correctly, the expected number of files."