CRM-14865 - prevent duplicate participant records when added offline.

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* CRM-14865: Prevent duplicate participants when using offline event registration
  https://issues.civicrm.org/jira/browse/CRM-14865
CRM-14865 - ensure contacts and participants arrays are keyed the same

To avoid:

Notice: Undefined offset: 1 in CRM_Event_Form_Participant->postProcess()
(line 1497...)

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* CRM-14865: Prevent duplicate participants when using offline event registration
  https://issues.civicrm.org/jira/browse/CRM-14865
Merge pull request #3501 from jmcclelland/no-dupe-participants

CRM-14865 - prevent duplicate participant records when added offline.