Protogen throwing "Item has already been added" exception on some proto file

[GoogleCodeExporter]

Hi,

when trying to generate C# code from some .proto file, protogen.exe is raising 
an exception "Item has already been added" for a key named "tmp".

What steps will reproduce the problem?
1. Download proto file 
https://developers.google.com/ad-exchange/rtb/downloads/realtime-bidding-proto.t
xt
2. Run code generation: protogen.exe -i:realtime-bidding-proto -o:tmp.cs 

What is the expected output? What do you see instead?
Expected: generated "tmp.cs" file.
Result: exception thrown by protogen.exe

What version of the product are you using? On what operating system?
Release 622, on Windows 7.

Please provide any additional information below.
None.

Original issue reported on code.google.com by caput.r...@gmail.com on 5 Mar 2013 at 10:30

[GoogleCodeExporter]

This seems to be somehow related to my specific machine configuration, as it 
works fine on other computers. Still a bug though, as it's an uncaught 
exception, but it might be hard to reproduce.

Original comment by caput.r...@gmail.com on 5 Mar 2013 at 6:06