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Hi there, @Bull1.
Thanks for visiting the Community. Adding customers with identical information isn't an option in QuickBooks Online. However, you can modify one of them so the system will allow you to save it.
For future reference, you can read through this help article: Create invoices in QuickBooks Online. It helps you learn the basic steps in creating an invoice for your newly added customers.
Let me know if you have additional questions. We're always here to guide you. Stay safe.