[fysiks]

The game doesn't overwrite files on purpose. If it was able to overwrite files, nothing would ever work correctly.

You need a plugin to force the client to download new files and then the plugin will tell the game to use that new file instead of the original. There are plugins out there that do this depending on what exactly you're trying to replace. For example, there are at least a couple plugins that do this for player models and I'm sure there are some out there for weapons too.