You've found another hardcoded limit: 4 ammos per weapon (GL already has 4)
As I said, it is hardcoded, and trying to increase it seems to cause a crash (see
bug 5283), we are investigating the issue
Edit:
Could also happen in case you made a backup of the original file without changing the extension. The game parses all .ufo files it can find.
Also, in case you took the e.g. weapons.ufo and use a copy of this file as /mods/mymod/myweapons.ufo this will also essentially end up in the game parsing the same file twice.
Just for the record: while the above is true — the game will parse the file twice — when the game finds more than one item with the same id, it will discard all definitions after the first one (it will still generate a lot of error messages of course)