Catch multiple entities with same name on XML import

[EJMFarrow]

• Prevent XML import if multiple inputs, PRTs or nodes within the same PRT have the same name (otherwise the first instance gets overwritten by the second).
• Applied to XML import only, not the API.
• It's still very easy to break things e.g. leaving an input name blank throws an Exception, submitting malformed XML complains there are no questions. Trying to catch everything gracefully would take a while...

[sangwinc]

Thanks @EJMFarrow