Set Field steps should look like this in your script editor:
Set Field [YourTable::YourField ; "X"]
You have two specify buttons with set field, use the first button to select the field where you want to enter the "x". Use the lower button to enter the expression of the value you want to set, the "X" in your case.
I appreciate it and was able to get the script to set the Global field as the ID field. It went through the entire script but didn't place an X in any field under the Mark column showing any duplicates.
I can't figure out how it knows to check the ID field of one record against the Global field in the previous record so it can note it as a duplicate.
Somethings off somewhere. I'm new to Filemaker but believe I'm doing it properly. There has to be something missing in the script that is posted on their knowledge base.
There are other approaches for working with duplicate values. Entering Find mode, putting an ! in the field that might store duplicates, performing the find and then sorting your records by this field so that duplicate values are in a group is another way to see which records might store a duplicate value instead of doing Show All Records.
This script works if you specify a sort order that sorts on the field, ClientID, that may have duplicate values. The sort groups all records with the same client ID next to each other. The global field is being used to compare the ID value of the previous record to the ID value of the current one. That won't work unless you've specified global storage for the global field. With today's FileMaker, you have a slightly faster/simpler option of using a variable instead of a global field for this, but the global field should still work for you if you have everything defined correctly.