Does running Google Photos in addition to the iOS Photos app cause two copies of each of my photos to be stored on my phone? I run both iOS' Photos app as well as the Google Photos app on my iPhone, but I worry that doing this causes two copies of each picture to be stored on my phone.  Does it, or do the apps share the photos they both show me?

I have an iPhone 6 running iOS 11.

Please note that this question is only about storage on the phone. What happens in the cloud as regards Google Drive and iCloud is not part of my question. In case it happens to be relevant, both apps back up my photos to their respective clouds.
The photos are stored on your device and Google accesses them there. 

It makes a copy of the selected photos (or all photos, depending on your settings) and uploads that copy to their servers for storage. 

You only have 1 copy of the actual photo and not your device but some space is also used by the Google app to keep the links between your photo and it's databases in sync.