xwiki-platform-web-templates is installed when declared as dependency of some extension

In this specific case, it does not make much sense for it to be a runtime dependency and is generally a mistake (see for example XWIKI-20964).

That being said, even when some extension makes the mistake, Extension Manager should see it's already there (like it does with more traditional core extensions). The difficulty is just that it's not packaged as a regular JAR but expended as part of the WAR content.