A first solution solution is to define
(setq org-refile-use-outline-path 'full-file-path)
you will get full file paths, however, this can be quite long:
and thus reduce readability.
A second solution, I prefer, is to use:
(setq org-refile-use-outline-path 'buffer-name)
in conjunction with the uniquify package.
Once configured with:
(setq uniquify-buffer-name-style 'post-forward-angle-brackets)
you will get (when refiling):
which it the best of two worlds: short but unambiguous refile targets.
Note: to make it works with helm (
M-x helm-mode) you must let Org generates all of the possible completions and present them at once thanks to:
(setq org-outline-path-complete-in-steps nil)