Add SSO domain error troubleshooting

docs/ce/troubleshooting/action-errors.mdx

@@ -32,3 +32,14 @@ Things to check:
         "job": "{\"projectName\":\"dev\",\"projectDir\":\".\",\"projectWorkspace\":\"default\",\"terragrunt\":false,\"commands\":[\"digger plan\"],\"applyStage\":{\"steps\":[{\"action\":\"init\",\"value\":\"\",\"extraArgs\":[],\"shell\":\"\"},{\"action\":\"apply\",\"value\":\"\",\"extraArgs\":[],\"shell\":\"\"}]},\"planStage\":{\"steps\":[{\"action\":\"init\",\"value\":\"\",\"extraArgs\":[],\"shell\":\"\"},{\"action\":\"plan\",\"value\":\"\",\"extraArgs\":[],\"shell\":\"\"}]},\"pullRequestNumber\":9,\"eventName\":\"issue_comment\",\"requestedBy\":\"motatoes\",\"namespace\":\"diggerhq/demo-opentofu\",\"runEnvVars\":{\"DEFAULT_BRANCH\":\"main\",\"PROJECT_DIR\":\".\",\"PROJECT_NAME\":\"dev\",\"PR_BRANCH\":\"motatoes-patch-7\"},\"stateEnvVars\":{},\"commandEnvVars\":{},\"state_role_name\":\"\",\"command_role_name\":\"\",\"backend_hostname\":\"https://c1db-5-151-139-93.ngrok-free.app\",\"backend_organisation_hostname\":\"digger\",\"backend_job_token\":\"cli:f4bfd7a5-ec11-4d43-ab88-93d12674e1d7\"}"
       },
 ```
+
+## DataFetchingError: Domain is already added
+
+This error occurs when trying to add a domain to your SSO configuration that has already been registered. Each domain can only be associated with one SSO provider configuration.
+
+**Solutions:**
+
+- Check if the domain is already configured in your SSO provider settings
+- If you need to update the configuration, remove the existing domain first before re-adding it
+- Verify you're not accidentally duplicating callback URLs in your SSO provider (Auth0, Okta, etc.)
+- Ensure the domain matches exactly (including protocol and trailing slashes where required)