Duplicate funtion can't work with constraints!

Hi,

I make some constraints on some fields of table (object) to make sure that these fields are unique. In the form view, there is a button 'Duplicate' to make a copy of existed record. But it failed due to these constraints.

So I wonder there are some ways to empty these unique fields when doing Duplicate action to avoid these constraints.

In my case, I make a constraint to avoid overlapping in time between contracts for the same employee.
1. contract 1: from 1/1/2011 to 31/12/2011
2. contract 2: must not be in the interval of contract 1. Maybe from 1/1/2010 to 31/12/2010, not 1/1/2011. Or maybe from 1/1/2012, not 15/12/2011.

When I do Duplicate action, this constraint is activated, so can't make a new record.

Thanks.