Static plugin loaded twice

Whilst migrating an existing Qt 5 application over to 6, I've encountered a problem with the plugin interface. This problem can easily be reproduced by building and running the Plug & Paint example (in widgets\tools\plugandpaint).

In the example, both a static and dynamic plugin are built and loaded in the application. In Qt 5 the behavior is correct and the plugins are only loaded once. In Qt 6 however, the static plugin is loaded twice, which is unwanted.

A workaround could be to implementing a filter on the 
`QPluginLoader::staticInstances();` results.