What OS is Duplicati on? Windows causes this on files and folders that end with space or period.
Constraint failed UNIQUE constraint failed: FilesetEntry.FilesetID, FilesetEntry.FileID looks similar.
Support problematic Windows paths #4256 avoids the issue, but change is not in any release yet.
You might have to try backing up your area adding a bit at a time until you can locate the bad name.
Another option would be to set up a log-file at log-file-log-level=verbose and search with a good tool.
Possibly you will need one that will let you find lines ending with a space or period. Finding folders is simpler because Duplicati logs folders with backslash at the end. Below is a test set of odd folders:
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend\
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend \
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend.\
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend\dir_withoutspace_subdir\
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend.\dir_without_space
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend\dir_withoutspace_subdir\
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend \dir_with_space
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend\dir_withoutspace_subdir\
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend\dir_without_space
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend\
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend \
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend.\
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend\dir_withoutspace_subdir\
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend.\dir_without_space
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.MetadataPreProcess.FileEntry-AddDirectory]: Adding directory C:\tmp\sub1\subend.\
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FilePreFilterProcess.FileEntry-CheckFileForChanges]: Checking file for changes C:\tmp\sub1\subend.\dir_without_space, new: True, timestamp changed: True, size changed: True, metadatachanged: True, 7/11/2020 6:31:48 PM vs 1/1/0001 12:00:00 AM
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.MetadataPreProcess.FileEntry-AddDirectory]: Adding directory C:\tmp\sub1\subend\dir_withoutspace_subdir\
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileBlockProcessor.FileEntry-NewFile]: New file C:\tmp\sub1\subend.\dir_without_space
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend\dir_withoutspace_subdir\
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\tmp\sub1\subend \dir_with_space
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.MetadataPreProcess.FileEntry-AddDirectory]: Adding directory C:\tmp\sub1\subend \
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FilePreFilterProcess.FileEntry-CheckFileForChanges]: Checking file for changes C:\tmp\sub1\subend \dir_with_space, new: True, timestamp changed: True, size changed: True, metadatachanged: True, 7/11/2020 6:31:25 PM vs 1/1/0001 12:00:00 AM
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.MetadataPreProcess.FileEntry-AddDirectory]: Adding directory C:\tmp\sub1\subend\dir_withoutspace_subdir\
2020-08-01 20:59:34 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileBlockProcessor.FileEntry-NewFile]: New file C:\tmp\sub1\subend \dir_with_space