Hi Experts

How to check ,

The worker already assigned for the particular task with some x leader for the particular date ,

Here am having the

Date(textfield) , Leader name(drop down), Worker name's(Multiple List box),

or is there any module for that ?

for example

x,y,z are the leader, a,b,c are the workers

assume a is assigned to x on 15-march-2009

if admin again try to assign the 'a' worker to y leader for the task , on 15-march-2009 ,

Then need to raise the validation error

Like the a is already assigned,please choose another worker ,

any grate idea