How will you avoid duplicate entries in the forms?

We can restrict duplicate entries in the form
cheak which coloum can be maintained uniqueness
for that coloumn item text
write a trigger when_key_navigate
declare
e_empno number(5);
cursor c is select * from emp;
begin
for i in e_emp loop
if e_empno= i.empno then
dbms_output.put_line("already exist");
end if;
end loop;
:empno=e_empno;
end;