Don't allow registering the same component twice.

django_components/component.py

@@ -1,8 +1,14 @@
+class AlreadyRegistered(Exception):
+    pass
+
 class ComponentRegistry(object):
     def __init__(self):
         self._registry = {}  # component name -> component_class mapping
 
     def register(self, name=None, component=None):
+        if name in self._registry:
+            raise AlreadyRegistered('The component %s is already registered' % name)
+
         self._registry[name] = component
 
 # This variable represents the global component registry