Try each of the following until the issue is resolved:
  • Enter a new account number that is not already in use
  • Renumber the existing duplicate account before adding the new account

If you are not able to find the account, verify that you are able to view inactive accounts.
  1. Go to Configuration > Business Rules
  2. Select General
  3. Under "Exclude inactive accounts from..", un-mark the checkbox for Searches
  4. Go back to Records > Account > Open Account and search for the account again