I was practicing firebase CodeLab (Friendly Chat) for learning firebase with android. In that tutorials ,I made a project on firebase console named as "Friendly Chat" and then I add my android app package name and SHA-1 so that invites and Google Auth could be work. After that tutorial get compeleted, I got firebase Auth demo project from github.I add auth demo project's package name and SHA-1 in same console project "Friendly Chat",its mean now console project "Friendly Chat" have two android apps.Then I downlaoded google.services.json file of Auth project and when I run project in Android Studio,Auth demo did not work as it keep showing loading progress Dialog.I thought it may be because I added two android apps in same console project so,I delete auth app from Friendly Chat console project and made another project on firebase console named as "Project Auth" and I add package name of Auth demo android project (which I downloaded from firebase github repository) but when I tried to add SHA-1 copied from "Friendly Chat" console project,it gives me below error
"An OAuth2 client already exists for this package name and SHA-1 in another project"
why Im getting this error even I have deleted Auth app package name and SHA-1 from other console project.
So the answer is well written in the error. You can't have two projects of same package name. Even if you delete it. It will take alteast 4-5 days to get deleted fully from developer's console.
So only solution is to generate a new SHA-1 key by custom signing the app by generating a signed apk from android studio. Or just create a new project with different package name both ways will work for now.
Hope this info helps. Do let me know if it changes anything for you.
EDIT: I encourage you to go through these official Google docs on signing apk to understand this process.