diff -r 000000000000 -r 6474c204b198 toolkit/mozapps/installer/find-dupes.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/mozapps/installer/find-dupes.py	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,48 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+import sys
+import hashlib
+from mozpack.packager.unpack import UnpackFinder
+from collections import OrderedDict
+
+'''
+Find files duplicated in a given packaged directory, independently of its
+package format.
+'''
+
+
+def find_dupes(source):
+    md5s = OrderedDict()
+    for p, f in UnpackFinder(source):
+        content = f.open().read()
+        m = hashlib.md5(content).digest()
+        if not m in md5s:
+            md5s[m] = (len(content), [])
+        md5s[m][1].append(p)
+    total = 0
+    num_dupes = 0
+    for m, (size, paths) in md5s.iteritems():
+        if len(paths) > 1:
+            print 'Duplicates %d bytes%s:' % (size,
+                  ' (%d times)' % (len(paths) - 1) if len(paths) > 2 else '')
+            print ''.join('  %s\n' % p for p in paths)
+            total += (len(paths) - 1) * size
+            num_dupes += 1
+    if num_dupes:
+        print "WARNING: Found %d duplicated files taking %d bytes" % \
+              (num_dupes, total) + " (uncompressed)"
+
+
+def main():
+    if len(sys.argv) != 2:
+        import os
+        print >>sys.stderr, "Usage: %s directory" % \
+                            os.path.basename(sys.argv[0])
+        sys.exit(1)
+
+    find_dupes(sys.argv[1])
+
+if __name__ == "__main__":
+    main()