display-buffer-reuse-frames is a variable defined in `window.el'.
Its value is

  • This variable is obsolete since 24.3;
    use a `reusable-frames' alist entry in `display-buffer-alist'.

Non-nil means `display-buffer' should reuse frames.
If the buffer in question is already displayed in a frame, raise
that frame.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.