ADDing new investment accounts in Quicken 2008 Delixe

Why do you think Quicken wants to "add" a new account when it is giving you the opportunity to tell it that there is an existing Quicken account for the real-world account?

Make a backup of your data; then complete the account setup, telling Quicken which real-world accounts match existing Quicken accounts, and which real-world accounts are new to Quicken. If you don't like the results, just revert to the backup.