Method 3 is the only way I know that you then can use color by field or conditions like you showed.
I’ve used it in the waaaay past in my total n00b days, before the Dedupe App, and it works. A lot of work and fields to get there though, but it givs live ‘commentary’ in your base without you running a script or app.
it depends on what kind of duplicate search you need
quick check for duplicates
change to Single select
sorted list of duplicates will be at the top
don’t press Save, press Cancel
Primary field can’t be single select, so one additional step needed
If you have large table and want to mark duplicates
Select whole column, ctrl+c/ctrl+V (to new column)
If you want to prevent entering existing value by automation - it depends on what kind of “prevention” do you need. Inform by mail alert, delete/rename… in forms, the only way to “validate” - is to create conditional required field, make it single select, put warning in description and delete all choices. Thus, user can’t submit until it disappears…