Finding duplicates in a large excel list is a tremendous hard work. Microsoft Excel is a perfect tool to solve this problem.In a new column, preferably next to the start value of your list, create a new formula: =IF(COUNTIF($A$1:$A$10,A1)>1,"Double","")
Copy that formula to all adjacent cells of your list.

Make sure that "$A$1:$A$10" in COUNTIF($A$1:$A$10,A1) covers the list range you want to check for duplicates.