CRM-15452 - ensure dupes not added when no event fee is used.

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* CRM-15452: When no registration fee is involved, duplicate registration still created for event
  https://issues.civicrm.org/jira/browse/CRM-15452
Merge pull request #4359 from jmcclelland/duplicate-registration-take-two

CRM-15452 - ensure dupes not added when no event fee is used.