Command classes creating errors

I want to have several different commands on my server. To organize these, I created a package “commands” in wich I created the classes. (And yes, I did register the classes.) One class for my custom /msg plugin works just fine. But when I create a nother class for a custom /help command, and error occurs in more or less the second line:
public class CustomHelpeCommand implements CommandExecutor {
It says, there is an error and the two command classes are conflicting with each other. Also, I cannot use onCommand methods in the plugin. These then also produce errors.
Anyone know, what to do?