BEGIN;

SELECT 
get_filename(path) AS fn, count(path) AS n
INTO TEMPORARY TABLE dup_files
FROM paths
GROUP BY get_filename(path)
HAVING count(path) > 1;

SELECT * FROM dup_files;

-- Creates UNIQUE index on PATH as well
ALTER TABLE dup_files ADD CONSTRAINT dup_files_path_pkey PRIMARY KEY (fn);

-- Now build your side-by-side table of duplicates:

SELECT p1.path, p2.path
FROM paths p1 INNER JOIN paths p2
     ON (get_filename(p1.path) = get_filename(p2.path))
WHERE EXISTS(SELECT 1 FROM dup_files WHERE fn = get_filename(p1.path))
  AND p1.path > p2.path
ORDER BY get_filename(p1.path), p1.path, p2.path;

ROLLBACK;