Does ShortPixel Website Optimizer re-optimize already optimized images?
ShortPixel Website Optimizer, which offers a straightforward web interface to our API available to any PHP website, can optimize images on any folder you tell it to: https://shortpixel.com/web-tool-docs
You can keep adding as many images as you want in the already optimized folders. When you press again the OPTIMIZE NOW button, your already optimized images will be skipped. No over-optimization! That's because ShortPixel keeps a small file in the same folder as the images are.
Do not delete the .shortpixel file if you want to preserve the data about previous optimizations.