Use set instead of list with ALL_CELERY_BROKERS and ALL_CELERY_BACKEN…

…DS (#375)

src/pytest_celery/defaults.py

@@ -38,20 +38,20 @@
 # Tests that do not rely on default parametrization will not be affected.
 
 
-ALL_CELERY_BACKENDS = []
-ALL_CELERY_BROKERS = []
+ALL_CELERY_BACKENDS = set()
+ALL_CELERY_BROKERS = set()
 
 if _is_vendor_installed("redis"):
-    ALL_CELERY_BACKENDS.append(CELERY_REDIS_BACKEND)
-    ALL_CELERY_BROKERS.append(CELERY_REDIS_BROKER)
+    ALL_CELERY_BACKENDS.add(CELERY_REDIS_BACKEND)
+    ALL_CELERY_BROKERS.add(CELERY_REDIS_BROKER)
 
 if _is_vendor_installed("rabbitmq"):
     # Uses Kombu
-    ALL_CELERY_BROKERS.append(CELERY_RABBITMQ_BROKER)
+    ALL_CELERY_BROKERS.add(CELERY_RABBITMQ_BROKER)
 
 # Memcached is disabled by default regardless of its availability due to its experimental status.
 if _is_vendor_installed("memcached") and False:
-    ALL_CELERY_BACKENDS.append(CELERY_MEMCACHED_BACKEND)
+    ALL_CELERY_BACKENDS.add(CELERY_MEMCACHED_BACKEND)
 
 # Worker setup is assumed to be always available.
 ALL_CELERY_WORKERS = (CELERY_SETUP_WORKER,)