Review code to ensure 'version' is not necessarily unique

@apdavison proposed that:
model version does not have to be unique, but rather the combination of version and parameters should be considered unique.

You can have two model instances with the same code version but different parameters.

I had always worked with the presumption that version for instances was similar to alias for models/tests (a human readable unique identifier, as opposed to the UUID). All my implementations were with this understanding, and I believe even the unit tests were written to test only this aspect. So parts of the codebase could be faulty for this scenario.

The code for the web apps and unit tests would need to be reviewed and updated, where required, to handle this.

To be decided: if the same rule should apply for test instances

Start by reviewing the issue's proposal and tracing how model version and parameters are handled across the web apps and unit tests; no specific files are named. Resolve whether the same uniqueness rule applies to test instances before changing behavior. Done means the relevant code and tests consistently handle identical versions with different parameters.