diff --git a/imagekit/models/fields/__init__.py b/imagekit/models/fields/__init__.py
index 1042d83..3f1a2f6 100644
--- a/imagekit/models/fields/__init__.py
+++ b/imagekit/models/fields/__init__.py
@@ -7,16 +7,18 @@ from ...registry import register
 
 
 class SpecHostField(SpecHost):
-    def set_spec_id(self, cls, name):
+    def _set_spec_id(self, cls, name):
+        spec_id = getattr(self, 'spec_id', None)
+
         # Generate a spec_id to register the spec with. The default spec id is
         # "<app>:<model>_<field>"
-        if not getattr(self, 'spec_id', None):
+        if not spec_id:
             spec_id = (u'%s:%s:%s' % (cls._meta.app_label,
                             cls._meta.object_name, name)).lower()
 
-            # Register the spec with the id. This allows specs to be overridden
-            # later, from outside of the model definition.
-            super(SpecHostField, self).set_spec_id(spec_id)
+        # Register the spec with the id. This allows specs to be overridden
+        # later, from outside of the model definition.
+        super(SpecHostField, self).set_spec_id(spec_id)
 
 
 class ImageSpecField(SpecHostField):
@@ -59,7 +61,7 @@ class ImageSpecField(SpecHostField):
             source = image_fields[0]
 
         setattr(cls, name, ImageSpecFileDescriptor(self, name, source))
-        self.set_spec_id(cls, name)
+        self._set_spec_id(cls, name)
 
         # Add the model and field as a source for this spec id
         register.source_group(self.spec_id, ImageFieldSourceGroup(cls, source))
@@ -92,7 +94,7 @@ class ProcessedImageField(models.ImageField, SpecHostField):
                 height_field, **kwargs)
 
     def contribute_to_class(self, cls, name):
-        self.set_spec_id(cls, name)
+        self._set_spec_id(cls, name)
         return super(ProcessedImageField, self).contribute_to_class(cls, name)
 
 
diff --git a/imagekit/registry.py b/imagekit/registry.py
index a28738f..620f9f8 100644
--- a/imagekit/registry.py
+++ b/imagekit/registry.py
@@ -15,7 +15,8 @@ class GeneratorRegistry(object):
         before_access.connect(self.before_access_receiver)
 
     def register(self, id, generator):
-        if id in self._generators:
+        registered_generator = self._generators.get(id)
+        if registered_generator and generator != self._generators[id]:
             raise AlreadyRegistered('The generator with id %s is'
                                     ' already registered' % id)
         self._generators[id] = generator
diff --git a/imagekit/specs/__init__.py b/imagekit/specs/__init__.py
index c12acfa..9a2ba66 100644
--- a/imagekit/specs/__init__.py
+++ b/imagekit/specs/__init__.py
@@ -4,7 +4,7 @@ from hashlib import md5
 import pickle
 from ..cachefiles.backends import get_default_cachefile_backend
 from ..cachefiles.strategies import StrategyWrapper
-from ..exceptions import MissingSource
+from ..exceptions import AlreadyRegistered, MissingSource
 from ..processors import ProcessorPipeline
 from ..utils import open_image, img_to_fobj, get_by_qname
 from ..registry import generator_registry, register
@@ -202,7 +202,17 @@ class SpecHost(object):
 
         """
         self.spec_id = id
-        register.generator(id, self._original_spec)
+
+        if self._original_spec:
+            try:
+                register.generator(id, self._original_spec)
+            except AlreadyRegistered:
+                # Fields should not cause AlreadyRegistered exceptions. If a
+                # spec is already registered, that should be used. It is
+                # especially important that an error is not thrown here because
+                # of South, which will create duplicate models as part of its
+                # "fake orm," therefore re-registering specs.
+                pass
 
     def get_spec(self, source):
         """