[ssrs-2008] In SSRS, why do I get the error "item with same key has already been added" , when I'm making a new report?

I just got this error and i came to know that it is about the local variable alias

at the end of the stored procedure i had like

select @localvariable1,@localvariable2

it was working fine in sql but when i ran this in ssrs it was always throwing error but after I gave alias it is fixed

select @localvariable1 as A,@localvariable2 as B