The Google Photos site is now starting to show an option to allow you to merge faces in your library. Photos since the beginning has had facial recognition built into it and has done a reasonable job of helping you identify people in your albums and tagging them. The challenge with it has come with age progression of children and photos of peoples at odd angles. It seems that the Photos team have be doing a bit of work on the Artificial Intelligence and Machine Learning algorithms on the feature.
On the Google Photos site, if you go to the Albums tab and click on People & Pets album, you will see a banner at the top of the page with suggestions of faces to merge under the tag for that person or pet.