@Mihai_Cog When you find an entry having same values in both pre_image and post_image, it means that record was indeed updated. But there was no change in values. This record went through the merge statement, updated the column values. We will not know whether it indeed changes the value or it's going to update the same value. All the update statement in Merge does is, to identify records eligible for update and update whatever value it has on the source table. 

If you only want to have real updates, I would suggest using a filter in your merge statement to eliminate records with the same value in the column you are interested.