Regardless, only one W-9 is required for the relationship. However, it takes just as long to sign that section on the card as it does write "W-9 on file".
Use caution though, the primary on the account must have a signed W-9 on file. If someone has a W-9 and then becomes secondary on another account, a W-9 for the primary on that account must be obtained or backup withholding is in order.
If you have been reporting interest for the customer in the past and he/she becomes subject to backup withholding, the IRS will promptly let you know.
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I wish that God grants you twice what you wish for me.