Duplicate entries for buffers-or-files produces duplicate items in result

[ahmed-shariff]

Related to discussion in #258

If there a duplicate entries in the buffers-or-files, they are not filtered out, resulting in duplicate entries in the result.

To reproduce:
if content of file1.org are:

* heading 1 in file1
* heading 2 in file1

(let ((file-name "file1.org"))
  (with-current-buffer (find-file-noselect file-name)
    (org-ql-select (list (current-buffer) file-name) '(level 1)
      :action (lambda () (substring-no-properties (org-get-heading t t t t))))
    ;;; result: ("heading 1 in file1" "heading 2 in file1" "heading 1 in file1" "heading 2 in file1")
    (org-ql-select (list file-name file-name) '(level 1)
      :action (lambda () (substring-no-properties (org-get-heading t t t t))))
    ;;; result: ("heading 1 in file1" "heading 2 in file1" "heading 1 in file1" "heading 2 in file1")
    ))

[alphapapa]

This doesn't seem like a problem to me. Why should it be the responsibility of org-ql-select to remove these duplicate filenames rather than that of the calling code? And doing it in org-ql-select would mean imposing that cost on every call, even when it's not necessary.

[ahmed-shariff]

Sounds good to me :)