If you attach a file with the same name (from inside the app), and it’s the same file – Anki recognizes that and doesn’t re-add the file to collection.media.
If you attach a file with the same name, and it’s a different file – Anki appends an identifying hash to the filename to make it unique in collection.media, like -9d14e6116a6b19e3a21b29a10edf36b7097216ca.