When I filed my federal income tax, I did not check off the box that I received the $600 stimulus check

Did you get it after you filed?  Does your 1040 line 30 have an amount on it?  Did you get both the 1200 and the 600 for the 2 rounds?  If it showed up again on line 30 the IRS will adjust if off for you and reduce your refund or send you a bill.  They know they already send it to you.  You don't have to do anything.  

Thank you so much! I was hoping that they would adjust my refund, being that they are the ones who sent me the Stimulus. I've been worrying needlessly.  Again, thanks!