Why am I getting a warning about a conflict with a thumbnail regenerate plugin?
ShortPixel optimizes images and all the plugins that are used to regenerate images (or have this option available like Simple Image Sizes plugin) can lead ShortPixel to optimize the images multiple times.
The solution to this is to deactivate ShortPixel when such a thumbnail regenerate plugin is active and used.
Once it has finished regenerating all the thumbnails, the plugin can be deactivated and ShortPixel can be activated.
In the case of Simple Image Sizes plugin, if just have it active but you don’t use it to regenerate thumbnails, it should be safe to just dismiss the warning message.