I have multi website setup and,
I have create one Custom Customer Attribute named “mobile_number”, that shows on create account form at frontend.
Now the issue is while creating customer in one website and customer registers with “123” mobile_number and then again if I create the customer with same mobile_number in second website then it gives me error of The value of the “Mobile Number” attribute isn’t unique. Set a unique value and try again.
Basically It has to be allow to use same mobile_number in both websites.