Hi @Easter1916 ,
The mystery is that it is currently duplicating successfully for one staff member and not for another.
Based on my tests, I could save the sent item both my sent item folder and the shared mailbox's sent item folder after setting the"Copy items sent as this mailbox". According to your description, the issue may be related to that user's client, so where is the sending item of this employee who cannot work normally is saved(in his mailbox or in shared mailbox)?
If it was saved in the shared mailbox only, as I know, the registry value "DelegateSentItemsStyle" in register editor may cause it, please kindly check if this value exists on the location below: HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences , if it exists, please try to remove it and restart your Outlook to check if the issue has any difference.
If that's not your case or the issue still exists, considering corrupted local data file may also cause it, it's recommended to working in online mode to for testing(File>Account settings>Account settings>uncheck the cached exchange mode), if it could work normally in online mode, we could try to repair your Outlook data file to check.
If that's invalid, in order to further avoid the interference of corrupted outlook profile, we could try to recreate a new Outlook profile and readd your account to test. For more information about how to create a new profile and how to switch to a new profile, please refer to this article: Overview of Outlook e-mail profiles
If your issue has any update, please feel free to post back.
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