So far we assumed that when you want to test MediaWiki and its extensions for a particular environment, you build a system from scratch, run your tests and assess if the new functionality is an improvement over the old. It turns out that the notion that a particular revision of an extension includes the whole of that extension is not sound. Several extensions can not be installed by specifying their revision.
The ability to test if an extension is complete, can be determined by comparing the difference between the functionality after an upgrade or an installation. To do this you have to be able to save an "old" environment. Another reason why the new duplicate function is useful, is because it allows us to speed up large scale automated testing
Syntax: duplicate MW_installation MW_installation_new