break-hardlink-on-save is a variable defined in `files.el'.
Its value is

Whether to allow breaking hardlinks when saving files.
If non-nil, then when saving a file that exists under several
names (i.e., has multiple hardlinks), break the hardlink
associated with `buffer-file-name' and write to a new file, so
that the other instances of the file are not affected by the

If `buffer-file-name' refers to a symlink, do not break the symlink.

Unlike `file-precious-flag', `break-hardlink-on-save' is not advisory.
For example, if the directory in which a file is being saved is not
itself writable, then error instead of saving in some
hardlink-nonbreaking way.

See also `backup-by-copying' and `backup-by-copying-when-linked'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.