When an application is processed for an existing applicant, the Blackbaud NetCommunity plug-in will create a duplicate record and then merge the two.  Check for business rules in The Education Edge that may be preventing the duplicate applicant from being created, and disable the business rules in order to process the application.

In The Education Edge:
1. Navigate to Configuration, Business Rules
2. Select Applicant
3. Uncheck Automatically check for duplicate applicants

This will allow the transaction to process.