Hello,
Try to reset Your Phone app and see if this resolves the issue.
- Click on Start and select Settings.
- Click on Apps.
- Under Apps and Features, search for Your Phone app.
- Click on Your Phone and select Advanced Options.
- Under Apps Permission, make sure Background Apps is enabled.
- Scroll down and click on the Reset button.
- When asked for confirmation, click Yes.
Just be sure you save any messages you want to another device before doing above steps, as they will all be wiped.
Hope this helps!
Referenced from https://forums.tomsguide.com/threads/why-do-some-messages-i-send-show-twice-like-i-sent-the-same-message-twice.211821/.
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