The site option called USER_SEND_MAX_USER_NAMES controls how many usernames can be sent due to duplicate records. Follow the steps below to change this.
  1. Select Setup > Site Options
  2. Click Go
  3. Keyword search (ctrl+F) USER_SEND_MAX_USER_NAMES
  4. Adjust the number
  5. Scroll back to the top of the page and click Save
If you do not have access to the site option please contact support and request that this option be changed.