Hi Shakeel,
The combination of package name and SHA1 must be unique across all Google projects. If you're getting this error, it means you have already added this combination to another Firebase or Google project. You'll need to find that project and delete the entry, which may be in the form of an OAuth 2.0 client ID, before you can add it to a new project.
Do not delete the whole project when you find it, as whole project deletions take 30 days but deleting the package name / SHA1 pair directly is near-instant. Write in to Firebase Support if you have any trouble with this process.
- Sam