Maven runs unit tests twice with clover plugin

I have this plugin code in my pom.xml. If I remove this plugin then Maven won't run unit tests twice. I just wanted to know which part of this plugin makes the unit tests run twice.

<plugin>
    <groupId>com.atlassian.maven.plugins</groupId>
    <artifactId>maven-clover2-plugin</artifactId>
    <version>3.0.4</version>
    <configuration>
        <licenseLocation>/location/to/clover.license</licenseLocation>
        <generateXml>true</generateXml>
        <generateHtml>true</generateHtml>
    </configuration>
    <executions>
        <execution>
            <phase>generate-sources</phase>
            <goals>
                <goal>instrument</goal>
            </goals>
        </execution>
        <execution>
            <id>main</id>
            <phase>verify</phase>
            <goals>
                <goal>instrument</goal>
                <goal>aggregate</goal>
                <goal>clover</goal>
            </goals>
        </execution>
        <execution>
            <id>site</id>
            <phase>pre-site</phase>
            <goals>
                <goal>instrument</goal>
                <goal>aggregate</goal>
            </goals>
        </execution>
    </executions>
</plugin>

The clover plugin forks a life cycle up to the test phase using the instrumented classpath.

Your standard life cycle will run the tests (typically using surefire in the test phase, but there could be multiple executions or other test plugins invoking tests in that phase)

Then clover comes along and asks for them to be run again.

Now for my standard warning:

• Code coverage modifies the byte code that is executed in ways that are legal under the JVM specification.
• Such transformations can result in the addition of synchronisation points while updating the coverage map.
• Such additional synchronisation points can restrict the reordering of operations that the JVM is able to do.
• There are other changes that are not threading related, but rather complex to explain.

If you only ever run your test cases with coverage turned on, you cannot trust a 100% pass rate as coverage can mask bugs

Conversely, if you never run with coverage turned on, any change in JVM architecture can highlight new bugs that you didn't know.

• Test passes with and without coverage turned on => probably ok

• Test passes with coverage turned on, fails when off => beware, bug here

• Test passes with coverage turned off, fails when on => if test is not timing critical then bug here, but if tests run on production environment bug may only show when -server fully optimises the hot paths

• Test fails with and without coverage turned on => bug

"Stephen's golden rule: Just run the damn tests twice"