in src/main/java/org/apache/maven/plugin/resources/remote/ModelInheritanceAssembler.java [618:624]
private static void mergeExtensionLists(Build childBuild, Build parentBuild) {
for (Extension e : parentBuild.getExtensions()) {
if (!childBuild.getExtensions().contains(e)) {
childBuild.addExtension(e);
}
}
}