The CRM will not allow users to create duplicate contacts in the system. If the contact has moved accounts, you can assign the contact to the New Account and then add the opportunity to the contact with the new account.
To move a contact, use the following steps:
1. Sign into the CRM
2. Find the Contact Name
3. Click "Edit" when in contact profile
4. Look for the "Account Name" field
5. Click the "Clear" button on the right hand side
6. Click on the green "+" sign
7. Search for the New Account
8. Click on the blue link for New Account
9. The existing contact is now associated with the New Account.