I checked with my team, and it looks like there isn’t a recommendation for how to solve this if you don’t know that the user will want to link user identities from the beginning. Once the user has authorized via Google or Facebook and we notice that the emails match, it’s too late to address this.
Sorry I don’t have a more helpful answer for you. I’ve made a note that this is problematic, and will be sure to update the docs in the future if we make enhancements to Realm user management that might improve this.