Raise duplicate key error only when key is accessed

This can be achieved by:

• Creating new DuplicateKey object storing the key and all templates that that key refers to.
• Adding DuplicateKey to the templates dictionary whenever a template is added to an already existing key.
• Raising an error if a DuplicateKey is accessed.
• Ensure that any entries with DuplicateKey are ignores/included when iterating over templates as appropriate.