Read the post carefully
When you launch TvT, the game reads script files in ALL the folders inside the main TvT folder, that includes the MODS folder, so the mod files are read even if the mods aren't installed.
So, for example TvT.exe will read the stock "tiger.script" in the game folders, and it will read the modded "tiger.script" in the MODS folder, both files containing some identical "classes", hence the error message "duplicate class" and the crash.
Mods for TvT have to be installed outside of the main folder, which involves a small amount of folder fiddling but the screenshot in the post linked above should clear things up