Currently there is none, meaning that if you save a replay that has the same name as another replay, it will get overridden.
I just spent a minute trying to find an old replay before I realised I titled it the same as the one I saved just now.
Godot has a function called "file_exists" which returns true if a file of the specified path exists or not.

I would really appreciate if a confirmation popup would appear in case of a file name conflict to prevent such situations in the future.
(I experienced this on 0.1.2, so if it has been fixed already I apologize, but I couldn't find any mention of it in the changelog.)

Thank you for this game, it's awesome.