Duplicate panel on right side of mail

Thanks for replying, Frankfiz, and that additional info, it really helps! Just so we make sure we mention this, if you are double-clicking the message, it is expected behavior to have the message open like that. Another thing you can check are some of these settings:

"Change how emails are shown

In the Mail app  on your Mac, do any of the following:

* Turn off conversations to ungroup messages: Choose View > Organize by Conversation. Email conversations are off if there’s no checkmark next to the command.

* Sort messages: Choose View > Sort By, then choose an attribute, such as From or Size, and a sort order.

* Show the message preview on the side or bottom of the Mail window: Choose View > Show Side Preview or View > Show Bottom Preview. 

* Show more details: Choose View, then choose details to show, such as Date and Time or Message Size (a checkmark indicates a detail is shown).

* Show messages with dark backgrounds (when using Dark Mode on your Mac): Choose Mail > Settings, click Viewing, then select “Use dark backgrounds for messages.”

* Resize the message list: Drag the bar that separates the message list and the message preview to the left or right.

Alternatively, you can use column layout to view messages. See Use column layout."

And that info is from this Apple resource: Read emails in Mail on Mac - Apple Support

If you are not able to use the info above to resolve this issue, use safe mode to start up your Mac as it does the following things:

• Verifies your startup disk and attempts to repair directory issues, if needed
• Loads only required kernel extensions
• Prevents Startup Items and Login Items from opening automatically
• Disables user-installed fonts
• Deletes font caches, kernel cache, and other system cache files

Log in to your user account and test the issue while in safe mode and then restart normally and test the issue again.

How to use safe mode on your Mac - Apple Support

Have a great day and thanks for using the Apple Support Communities!

Hey there Frankfiz,

Thanks for reaching out to the Apple Support Communities.

To get started, when you say that you're seeing a duplicate panel, are you seeing another tab in the Mail app, similar to an open tab in Safari? If so, you should be able to close this extra tab by hovering your mouse over the tab, and clicking the Close button.

For more details, check out this link here: Use tabs in Mail on Mac - Apple Support

You can open multiple tabs in the Mail viewer window, to easily switch between different mailboxes or emails.

In the Mail app  on your Mac, do any of the following:

* Open a new tab in the Mail window: Press and hold the Shift key, then choose File > New Viewer Tab. Or choose View > Show Tab Bar, then click the New Tab button . 

* Show all tabs: To show thumbnails of all your open tabs, choose View > Show All Tabs. In the tab overview, click a tab or click  to open a new tab in the Mail window. To return to the current tab, choose View > Exit Tab Overview.

* Move between tabs: Click a tab, or press Control-Tab or Control-Shift-Tab to go to the next or previous tab.

* Reorder tabs: Drag a tab left or right.

* Close a tab: Move the pointer over the tab, then click the Close button .

* Close all other tabs: Move the pointer over the tab you want to keep open, then Option-click the Close button .

Let us know if that's able to help.

Best regards.