Secret already exists in Vercel
If your Vercel sync starts failing and the status shows this message:
Another Environment Variable with the same Name and Environment exists in your project.
then the problem you're facing is that one of the secrets in the Doppler config you're trying to sync has already been manually added to the Vercel project you're trying to sync to. Rather than allowing this to be overwritten, Vercel throws an error.
To solve this problem, browse to the Settings > Environment Variables area for the project in Vercel's dashboard. Any variables created by Doppler will have a Doppler logo next to them. Look for variables without that logo that also appear in your Doppler config. After you remove them, the sync should be able to complete successfully.