add rule class

Author: rawls238

graphql/core/validation/rules.py

@@ -148,7 +148,22 @@ def message(field_name, type):
 
 
 class UniqueFragmentNames(ValidationRule):
-    pass
+    def __init__(self, context):
+        super(UniqueFragmentNames, self).__init__(context)
+        self.known_fragment_names = {}
+
+    def enter_FragmentName(self, node, *args):
+        fragment_name = node.name.value
+        if fragment_name in self.known_fragment_names:
+            return GraphQLError(
+                self.duplicate_fragment_name_message(fragment_name),
+                [ self.known_fragment_names[fragment_name], node.name]
+            )
+        self.known_fragment_names[fragment_name] = node.name
+    
+    @staticmethod
+    def duplicate_fragment_name_message(field):
+        return 'There can only be one fragment named {}'.format(field)
 
 
 class KnownFragmentNames(ValidationRule):