How to use of the Maven Enforcer plugin?

I'd like to use the Maven Enforcer plugin to check to see if I have duplicate classes on my path.

I've tried the example from here.

But when I run it like this:

mvn enforcer:enforce

I get this error:

Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:1.0.1:enforce (default-cli) on project datapopulator: The parameters 'rules' for goal org.apache.maven.plugins:maven-enforcer-plugin:1.0.1:enforce are missing or invalid

Is there a way to use this correctly?

EDIT #1

If changing my config to this:

        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-enforcer-plugin</artifactId>
            <version>1.0.1</version>
            <executions>
                <execution>
                    <id>enforce-versions</id>
                    <goals>
                        <goal>enforce</goal>
                    </goals>
                    <configuration>
                        <rules>
                            <AlwaysPass />
                        </rules>
                        <fail>true</fail>
                    </configuration>
                </execution>
            </executions>
        </plugin>

Produces the same error.

The reason why your first version did not work is because there is a difference between a plug-in configuration inside the execution tag and a plug-in configuration outside the execution tag. The execution is only used when your plug-in is triggered by a special phase of the complete Maven build.

The Maven guide to configuration explains it better:

Configurations inside the tag differ from those that are outside in that they cannot be used from a direct command line invocation. Instead they are only applied when the lifecycle phase they are bound to are invoked. Alternatively, if you move a configuration section outside of the executions section, it will apply globally to all invocations of the plugin.