Remove Duplicate photos - how does it work?

[kiddingguy]

On my iPhone/iCloud have have duplicate photos.

There is an option to 'merge' these pictures. In such a way that the high res picture remains, and the other photo(s) are merged/deleted with "all relevant info".

How does this work?
Example: I have 2 photos; 1 on photo roll, the other in iMessage. The one in photo roll has the highest quality, and will thus remain. But what happens with the deleted picture? From what location will this be deleted? And will I be able to get to that picture anyhow?

Also for e.g. WhatsApp: I have an album with all my WhatsApp images. I guess it will be deleted here? But will this image remain in the initial conversation in WhatsApp.

Or, is iOS that smart that some tags will be given to this photo, that - based on that - the (meta)information will remain and the iOS/app is smart enough to locate the original picture from all the information that was in the deleted picture?