Only set default app if there isn't one already

celery/fixups/django.py

@@ -15,6 +15,7 @@ from datetime import datetime
 from importlib import import_module
 
 from celery import signals
+from celery.app import default_app
 from celery.exceptions import FixupWarning
 
 __all__ = ['DjangoFixup', 'fixup']
@@ -48,7 +49,8 @@ class DjangoFixup(object):
 
     def __init__(self, app):
         self.app = app
-        self.app.set_default()
+        if default_app is None:
+            self.app.set_default()
         self._worker_fixup = None
 
     def install(self):