Hi Thcymru,
Thanks for reaching out in Apple Support Communities.
We understand that your emails are being duplicated, on your iPad Pro. This is definitely unexpected behavior. To clarify, are you using the native Mail app? Also, does this occur with your Inbox, Sent box or both?
In the meantime, the following steps may help to resolve this issue:
1. Restart your iPad. This may seem like a simple step to take but it refreshes your device and the software.
2. Update iPadOS. Completing this step ensures that your device has the latest fixes. (Secure your data with a backup)
Let us know if this helps.
We look forward to your reply.