There has been a change in the animation associated with dragging-and-dropping emails in the macOS Mail app, it is different in Golden Gate than in Tahoe. In macOS 27.0, emails were 'doubled up' like in your screenshot when being dragged. That appears to have been fixed (for me, at least) in macOS 27.0.1.
There is still an animation component to the process that was not present in Tahoe. When selecting one email and first starting to drag it, there are two email icons associated with the cursor:
However, those two icons quickly resolve into a single icon:
When selecting >1 email, the initial animation is n + 1 icons (select two and start to drag, you see three email icons, select three and you see four, etc.).
Still, they quickly resolve into a single icon with a red badge showing the correct number of selected emails.
If you are seeing something different, try the following troubleshooting steps.
Have you rebooted your Mac? If not, that's the first step. If that does not help, then see if the problem is present when you boot in safe mode. That process performs some system cleanup and clears caches, and also disables 3rd party extensions that may cause problems.
Start up your Mac in safe mode - Apple Support
If that does not correct the issue, test in a new user account. A newly-created user account will have all settings at default, and if the issue does not occur in that new account then it provides guidance on what may need to be changed to correct the issue.
Add a user or group on Mac - Apple Support
Once the testing is complete, the new account can easily be deleted.
Delete a user or group on Mac – Apple Support
Those steps will help to isolate the problem.
It might help to download and run EtreCheck, written by a long-time forum member. The free version is fine. Post the full report using the Additional Text button. No personal information is revealed.
If the issue persists in safe mode and a new user account, that suggests that it's a bug that Apple needs to address. Bugs can be reported to Apple here:
Feedback - macOS - Apple
