Fix adding duplicate officers to incidents

[sea-kelp]

Description of Changes

Merging fix OrcaCollective#347 upstream.

Original PR description:
Only add new officers to incidents.

When a user edits an incident currently, we retrieve the Incident from the database and append all officers in the form to that Incident. This led to sqlalchemy trying to insert duplicate rows into the officer_incidents association table.

As far as I can tell, this was silently ignored by sqlalchemy in the past and no unexpected rows were created. With the upgrade to sqlalchemy 2.0.19 (OrcaCollective#344), the library appears to have started raising exceptions related to this behavior.

This change checks to see whether an officer already exists in Incident.officers before adding them so as to not add duplicate officers.

Notes for Deployment

N/A

Screenshots (if appropriate)

N/A

Tests and linting

• This branch is up-to-date with the develop branch.
• pytest passes on my local development environment.
• pre-commit passes on my local development environment.

[michplunkett]

Looks good to me! If possible, it'd be nice to have a migration that removes any duplicates, though I'd also be happy to write that after this is merged.