Prevent multiple drop masks added to drop containers.
Prevent multiple drop masks added to drop containers.

thorgate/django-upthor,Jyrno42/django-upthor,amPerl/django-upthor,thorgate/django-upthor,Jyrno42/django-upthor,Jyrno42/django-upthor,amPerl/django-upthor,amPerl/django-upthor,thorgate/django-upthor