Fix the LMSCourse backfill from grouping migration

The existing LMSCourse has two assumptions, with one unique index for
each:

- The same course (same authority_provided_id) can belong to multiple
  applications instances but we'll keep only one row in LMSCourse for it
  (unique h_authority_provided_id)

- authority_provided_id is generated from guid, lti_context_id, then,
  those two columns should also be unique together.

There's however a case when this is not true:

- Applications instance with a null GUID launches and creates a course
  (None, CONTEXT_ID). This generates a hashed authority_provided_id.

- The same application instance (ie, same row in ApplicationInstance) starts sending a GUID.
  We calculate a new (GUID, CONTEXT_ID) hash creating a new course, with a different authority_provided_id.

When we join with ApplicationInstance for the backfill we only have the
most recent GUID and we end up creating (GUID, CONTEXT_ID) duplicates.

The solution is to exclude this type of duplicate, which is safe
because this only happens on:

- GUID changes from null to not null. We don't allow other value changes
  for GUID.

- We only deduplicate courses when we have a newer version of the course,
  the older one can no longer be launched, the ApplicationInstance now has a GUID and it will sent along with every launch.

lms/migrations/versions/f61cb94edfc8_backfill_lms_course.py

@@ -13,21 +13,26 @@ def upgrade() -> None:
         sa.text(
             """
         WITH backfill as (
-            -- Deduplicate "grouping" courses on authority_provided_id
-            SELECT DISTINCT ON (authority_provided_id)
-                "grouping".created,
-                "grouping".updated,
-                tool_consumer_instance_guid,
-                authority_provided_id,
-                lms_name,
-                lms_id
-            FROM "grouping"
-            -- join on application_instances to get the GUID
-            JOIN application_instances on application_instances.id = "grouping".application_instance_id
-            -- Pick only courses, not sections or groups
-            WHERE grouping.type ='course'
-            -- Pick the most recent "grouping" there are duplicates
-            ORDER BY authority_provided_id, "grouping".updated desc
+            -- Deduplicate on guid, lms_id
+            SELECT DISTINCT on (tool_consumer_instance_guid, lms_id) *
+            FROM (
+                -- Deduplicate "grouping" courses on authority_provided_id
+                SELECT DISTINCT ON (authority_provided_id)
+                    "grouping".created,
+                    "grouping".updated,
+                    tool_consumer_instance_guid,
+                    authority_provided_id,
+                    lms_name,
+                    lms_id
+                FROM "grouping"
+                -- join on application_instances to get the GUID
+                JOIN application_instances on application_instances.id = "grouping".application_instance_id
+                -- Pick only courses, not sections or groups
+                WHERE grouping.type ='course'
+                -- Pick the most recent "grouping" there are duplicates
+                ORDER BY authority_provided_id, "grouping".updated desc
+            ) unique_by_authority_provided_id
+            ORDER BY tool_consumer_instance_guid, lms_id, unique_by_authority_provided_id.updated desc
         )
         INSERT INTO lms_course (
              created,