Unique Field on User Profile

Hi, We’re building a membership only site. Members are issued with a Membership Cards prior to registration which have a number between 100000 and 160000. We’ve used your plug-in with ACF to add a ‘Validation Field’ to the registration form and the validation for the Number Field range works perfectly. However we also need the value to be unique, so that two Members cannot register with the same Membership number. We have selected ‘Unique Globally’ but this is not preventing multiple users registering with the same Membership Number. Any help would be appreciated, Miles https://wordpress.org/plugins/validated-field-for-acf/