How to make it so that GraphQL doesn't allow the same email?

when I create and submit a user to the database, GraphQL allows the same email to be used multiple times. I'm learning through a tutorial and the guy in the vid got an error when using the same email (using the localhost/graphql to test it out). I am not receiving the same error. I've checked and we use the same version of GraphQL. Here's my mutation:

  name:'Mutation',
  fields: {
    addClient: {
      type:ClientType,
      args: {
        name: {type:GraphQLNonNull(GraphQLString)},
        email: {type:GraphQLNonNull(GraphQLString)},
        phone: {type:GraphQLNonNull(GraphQLString)},
      },
      resolve(parent, args){
        const client = new Client({
          name:args.name,
          email:args.email,
          phone:args.phone
        })

        return client.save()
      }
    }
}