Hello Grant Rowan,
Good day. Thanks for posting in the community.
The reason that the 2nd account gets a copy of the sent items from the Google Workspace account is for some email providers is that the server automatically creates an item in the Sent Items and Outlook uploads the same item again. For Read/Unread status of an email Outlook relies on the server. If the status from server is unread Outlook sets it accordingly. To work around the issue, select your device and follow the steps to turn off saving sent items in an email account.
On the File menu, select Account Settings > Account Settings.
Choose the IMAP, and then select Change.
Choose the Don't save copies of sent items check box.
Select Next, then Done.
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Md Ashabul Haque | Microsoft Community Moderator