Fix duplicate subject validation on secondary course selection #9193

Job	Run time
Build	3m 4s
Lint (rubocop)	1m 35s
Test (unit_models)	3m 17s
Lint (erblint)	1m 21s
Test (unit_services)	1m 24s
Lint (dfe_analytics)	51s
Lint (yarn_lint_and_test)	1m 4s
Test (unit_forms)	1m 11s
Deployment To Review v2	0s
Test (unit_components)	1m 17s
Test (unit_other)	1m 54s
Test (integration_auth)	1m 11s
Test (integration_find)	1m 12s
Test (integration_publish)	2m 23s
Test (integration_support)	1m 16s
Test (integration_other)	1m 10s
Parallel deployment before production v2	0s
Production deployment v2	0s
Sandbox deployment v2	0s
	24m 10s

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