I know how to setup reseller in WHM, but some how, I don't think it's proper..

Right now, there can't be the same username on the server. So if one reseller created a username name "tom", other resellers wouldn't be able to create that user because of the same username conflict.

Are there anyway to stop this from happening?

Thanks,
-Ken

No, its the way it is. Thats a good thing if you ask me..

I suggest to tell your reseller to think in another username :)

There's no way around this, as those usernames are used globally on the server. We've never had customers complain about this - If a username is taken, they simply come up with a new username for that account :).