Merge into - Halloween problem

I have a very similar problem as in the topic below.
Copy Specific document in same collection

So I will simply clone the docs in the same collection, will remove _id field and update some other fields. I don’t want to bring data to the server and insert afterwards.

How to prevent Halloween problem described here by following @steevej approach.

Is it a good approach to set unique indexes and expect potential fails after re-read and re-insert cases using whenMatched: “keepExisting”

Your use-case is not clear.

Share the code you have in mind and we can see if there is any pitfalls.