Hello, sam234. We understand how frustrating that can be. I’d like to clarify the issue you encountered with estimates being sent twice. Did your customer receive the estimate twice, or did you also receive a copy when sending it? Knowing this will help us determine the cause.
If you received a copy of the estimate, it's possible that you checked the "Email a copy at" feature in your QuickBooks Online settings. To prevent this from happening, you need to uncheck that option and then click Save.
For visual reference, please see the screenshot.


However, if your customer received the estimate twice, I suggest checking the CC/BCC fields. This could also be the reason they are receiving duplicates.


Let us know if you have more questions or need further assistance.