Depending on what you need to do with the data afterwards you can use vlookup, sumproduct, or filter unique in Excel.
=vlookup(A1,$D$2:$D$2000,1,false) This lets you maintain the two lists separately and compare them to each other.
=sumproduct(--($A$2:$A$2000=$A2)) where the array is your combined list, this would display the number of times it appears in the list total.
and toolbar data > filter and check unique to just dedupe the combined list if thats all you need.