Obviously Duplicati should not trip this way, though there are others (such as locked files) that can happen due to OS limitations. For example, locked files might require snapshots in order to be read by some other program.
If you don’t have a specific desire to backup the Duplicati databases, try something like –snapshot-policy=On to workaround Duplicati’s issue. If you do have a specific intent to be be able to restore Duplicati databases, make another job so that you can backup while neither the database nor the destination are in the middle of updates.
Does it make sense to backup Duplicati config and db files? raises that question, but the answer is your choice.