I did lot of debugging and tried every alternative.

I was creating validator in actionscript code instead of in mxml.

   1: private function doValidation(comp:TextInput):void{

   2:     var strValidator:StringValidator = new StringValidator();

   3:     // validator init code

   4:     strValidator.source = comp;

   5:     

   6:     strValidator.validate();

   7: }

Every time i called doValidation(comp), duplicate error message used to appear as error tooltip on comp control.

Finally after debugging framework code, i found that validation event handler method in UIComponent used to cache the validators assigned to controls.

To resolve this issue, either create the validator in mxml or reuse the validator instead of creating it for each method call.

   1: private var strValidator:StringValidator = new StringValidator();

   2:  

   3: private function doValidation(comp:TextInput):void{

   4:     // reset strValidator properties for reuse

   5:     strValidator.source = comp;

   6:     strValidator.validate();

   7: }

Now as we are using same instance of validator, duplicate error string issue should have been resolved.

http://manikmagar.wordpress.com/2009/02/25/move-items-between-flex-list-objects/