Added a unique index requirement to the text field of each statement.

This change should prevent any possible duplicate key issues.

chatterbot/adapters/storage/mongodb.py

@@ -29,6 +29,9 @@ def __init__(self, **kwargs):
         # The mongo collection of statement documents
         self.statements = self.database['statements']
 
+        # Set a requirement for the text attribute to be unique
+        self.statements.create_index('text', unique=True)
+
     def count(self):
         return self.statements.count()