Catch error and provide message

app/controllers/omniauth_callbacks_controller.rb

@@ -4,6 +4,9 @@ class OmniauthCallbacksController < Devise::OmniauthCallbacksController
   def openid_connect
     OidcAccount.link(spree_current_user, request.env["omniauth.auth"])
 
+    redirect_to admin_oidc_settings_path
+  rescue ActiveRecord::RecordNotUnique
+    flash[:error] = t("devise.oidc.record_not_unique", uid: request.env["omniauth.auth"].uid)
     redirect_to admin_oidc_settings_path
   end
 

config/locales/en.yml

@@ -321,6 +321,7 @@ en:
         send_instructions: "You will receive an email with instructions about how to confirm your account in a few minutes."
     oidc:
       failure: "Could not sign in: %{error}"
+      record_not_unique: "%{uid} is already associated with another account"
   home_page_alert_html: "Home page alert HTML"
   hub_signup_case_studies_html: "Hub signup case studies HTML"
   hub_signup_detail_html: "Hub signup detail HTML"