This happens because Duplicati runs through the data twice: once for counting the files and sizes, and then for the actual backup.
The error itself happens because the value returned from Photos is outside the supported date range (in the year 0000). This has been fixed in the latest canary where we treat such a value as the lowest supported value (in the year 0001):
It will be included in the next patch stable release (2.3.0.2).