Handle dupe prices during migration

[mbertrand]

What are the relevant tickets?

N/A

Description (What does it do?)

Fixes a migration failure caused by old unpublished runs with duplicate prices.

How can this be tested?

On the main branch:

• Run ./manage.py migrate learning_resources 0070_learningresource_location
• Assign duplicate prices to an existing resource and run:

from learning_resources.models import *
course = LearningResource.objects.filter(resource_type="course").exclude(prices=[]).first()
course.prices = [0, 0, 999]
course.save()
run = course.runs.first()
run.prices = [0, 0, 999]
run.save()

• Run ./manage.py migrate learning_resources, it will fail due to a duplicate key.
• Switch to this branch, run the above migration again. It should pass.

Additional Context

ETL pipelines currently ensure that prices for any particular resource and run are unique, but RC and prod have some old unpublished runs still around from before this was implemented, causing a migration error.

[shanbady]

👍