This was resolved in the Raiser's Edge version 7.96, which was released in November 216 and applied to databases in Blackbaud Hosting Services.  If your database is installed locally, download and install the latest version and patch, if applicable.  

If you are experiencing this issue and you are using this version or a later version, click Chat with Support, reference this article, and provide the following information so we can assist you:   
  • Examples of some of the affected Constituent IDs
  • Backup of the database
  • Version of Raiser's Edge

OR
  1. Create a new blank constituent with just a Last Name (and any other required fields) 
  2. Merge one of your existing records into this new record (don't choose the option to delete the source constituent)
  3. Repeat steps 1 and 2 for the second of the constituent records that needs to be merged (this will result in a total of four records for this person)
  4. Once you have verified that these two merges processed successfully, delete the original two records
  5. Merge the two new records together.