Open very old file, stuck in "Opening file", can't open any other files after that

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Open very old dupe (Adv. Duplicator 1)
2. Fail to open the file (unknown duplication format)
3. Try to open any other file

What is the expected output? What do you see instead?
Expected to get uncorrupted, working file to open.

Gives me the error "Already opening file" forever after that.
This is only fixed by restarting GMod.

What revision of Advanced Duplicator 2 are you using?(Don't put latest
version, put the revision number)
173

What Operating System is the server running?
Happens in many diferent servers, probably not relevant (also, I have no way of 
knowing what operating system the servers are running)

Please provide any additional information below.

I can attach a very old dupe if it's needed, for reproducing the error.
The error in console when I try to open an old file is:

[ERROR] addons/advdupe2/lua/advdupe2/sh_codec.lua:468: unknown duplication 
format
  1. error - [C]:-1
   2. Decode - addons/advdupe2/lua/advdupe2/sh_codec.lua:468
    3. InitializeUpload - addons/advdupe2/lua/advdupe2/cl_networking.lua:241
     4. DoClick - addons/advdupe2/lua/advdupe2/file_browser.lua:421
      5. OnMouseReleased - lua/vgui/dlabel.lua:206
       6. unknown - lua/vgui/dmenuoption.lua:116

Original issue reported on code.google.com by muteca...@hotmail.com on 20 Jul 2013 at 2:41