Thank you for replying and letting us know: Glad you no longer hhave that problem-situation!
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Re: "Now I'm trying to work out how to stop Mail from sending my deleted emails to archive"
You may need to check your settings to see (or change) where Mac is directed to send deleted emails.
In the Mail app: Try Choosing (left on top menu) "Mail" Then Settings > Accounts > Mailbox behaviours
If you have more than one provider (Eg: Yahoo , GMail etc.) there's a setting for each of them
(Note: Your MacOS Version may be newer than mine and may look a bit different than screenshot below)

NB: Also:
Click "Viewing" (shown in screenshot's top row): See "Move discarded messages to" (Bin or Archive)
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Re: "... and why it is copying emails"
Hard to guess ... Eg: If it is copying emails that you send ... (one more than the one in "Sent" Mailbox)
You may have a setting ticked for Mac to send you a copy to your Inbox ?
Mail > Composing : Make sure the box indicated (by red arrow) is not ticked (If ticked / click it to untick.

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Re: Resources : they don't help: the instructions don't line up with what I see on my screen.
Mmmm ... We don't know for sure which MacOS version you are using on your MacBook Air:
You can check by clicking the black Apple icon in top menu bar > "About this Mac"
For accurate info: You may need to select your MacOS version at the top of any articles you consult.