Hello,
To my knowledge, this is standard behaviour. There is no standard control for preventing having the same phone number multiple times on the same BP.
If you need to restrict this, you will have to do some custom development.
As phone numbers are possibly maintained in different views, I would suggest enhancing GenIL implementation class to avoid the possibility of adding an already existing phone number to a BP.
Hope this helps.
Best regards,
Sylvain AGUETTAZ