FIx _ScheduleTypeManager

Raise a warning instead of an exception when a duplicate import is found.
Additionally log the error.

src/core/schedules/base.py

 import logging
 from abc import ABCMeta, abstractmethod
 from datetime import timedelta
+from warnings import warn
 
 from django.conf import settings
 from django.utils import timezone
@@ -172,7 +173,8 @@ class _ScheduleTypeManager:
 
             if klass_ident in self.types:
                 if self.types[klass_ident] == klass:
-                    raise RuntimeWarning(f'Duplicate import of {line} for schedule support "{klass_ident}".')
+                    warn('Duplicate import of %s for schedule support "%s".', line, klass_ident)
+                    self._logger.error('Duplicate import of %s for schedule support "%s".', line, klass_ident)
                     continue
 
                 raise ImportError(f'The identifier "{klass_ident}" of "{line}" is already in use by "{self.types[klass_ident]}".')