SQL & PL/SQL :: Creating Composite Unique Key With Only 1 Column Allowed To Be Changed

I have a table say MY_TAB with columns as below

emp_id number,
name varchar2(30),
from_dt date,
remarks varchar2(60)

insert into MY_TAB values (1,'TOM','01-JAN-13', 'some remark');
insert into MY_TAB values (1,'TOM','02-JAN-13', 'some remark');
insert into MY_TAB values (2,'TOM','01-JAN-13', 'some remark');
insert into MY_TAB values (3,'TOM','01-JAN-13', 'some remark');
insert into MY_TAB values (4,'TOM','01-JAN-13', 'some remark');
insert into MY_TAB values (4,'TOM','02-JAN-13', 'some remark');

How do I ensure that when a user tries to insert record with emp_id as 1, then he should only be allowed to enter another from_dt but the value in the name column have to be the same as in the previous row of emp_id 1.

insert into MY_TAB values (1,'TOOM','03-JAN-13') --shld not be allowed.