Keycloak 8: User with username ‘admin’ already added

Just to clarify the other answers. I had the same issue. What helped for me was:

1. stop all containers

2. comment out the two relevant lines

   version: "3"
   
   services:
     keycloak:
       image: quay.io/keycloak/keycloak:latest
       environment:
         # KEYCLOAK_USER: admin
         # KEYCLOAK_PASSWORD: pass
         ...
   

3. start all containers;

4. wait until keycloak container has successfully started
5. stop all containers, again

6. comment back in the two lines from above

   version: "3"
   
   services:
     keycloak:
       image: quay.io/keycloak/keycloak:latest
       environment:
         KEYCLOAK_USER: admin
         KEYCLOAK_PASSWORD: pass
         ...
   

7. start all containers

This time (and subsequent times) it worked. Keycloak was running and the admin user was registered and working as expected.