Hi Nitin
I guess you are trying to enter add line and then entering the personal number which is not required in this. Add line to used to add a new number to be maintained which is why the message says that number already exists.
Instead, please select the number and click on delete values which will delete the master data values.
Try this and let us know if it doesn't work. we will analyse and try something else.
Regards
Karthik