Fixed #9. COrrectly uniquify slugs.

taggit/models.py

 from django.contrib.contenttypes.models import ContentType
 from django.contrib.contenttypes.generic import GenericForeignKey
-from django.db import models
+from django.db import models, IntegrityError
 from django.template.defaultfilters import slugify
 
 
@@ -12,9 +12,17 @@ class Tag(models.Model):
         return self.name
     
     def save(self, *args, **kwargs):
-        if not self.pk:
-            self.slug = slugify(self.name)
-        super(Tag, self).save(*args, **kwargs)
+        if not self.pk and not self.slug:
+            self.slug = slug = slugify(self.name)
+            i = 0
+            while True:
+                try:
+                    return super(Tag, self).save(*args, **kwargs)
+                except IntegrityError:
+                    i += 1
+                    self.slug = "%s_%d" % (slug, i)
+        else:
+            return super(Tag, self).save(*args, **kwargs)
 
 
 class TaggedItem(models.Model):

taggit/tests/tests.py

@@ -61,6 +61,10 @@ class AddTagTestCase(BaseTaggingTest):
         f = Food()
         with self.assert_raises(ValueError):
             f.tags.all()
+    
+    def test_unique_slug(self):
+        apple = Food.objects.create(name="apple")
+        apple.tags.add("Red", "red")
 
 
 class LookupByTagTestCase(BaseTaggingTest):