Recursively search directories for a filename, stop descending if found

I have a directory-tree which contains files with the same name, e.g.:

a/file
a/...
a/b/file
x/a/file
x/a/...
x/b/file

I'm trying to use find (with -exec and -prune) to get all paths where file occurs first when recursively searching the tree. My example would return:

a/file
x/a/file
x/b/file

It does not return a/b/file because a file called file is already in a/. How can I achieve that with Bash?