It appears that there’s no way to query any of the duplicati binaries to return just a version string. Such a command is typically expected by users, and is helpful for diagnostics, as well as scripts.
For example, a log analysis script might check what version of duplicati is installed to determine what regular expressions to use when analyzing the logs, if the format changes between versions - or to decline to run on versions which do not generate logs with required details. It would probably also be of use to those who set up centralized configuration management utilities.