Update 4.7.22 : Operation failed: Update Indices DB Error: already exists
There are a couple of posts on this issue. The one that worked easiest for me is the artfulrobot answer in How to find and/or fix missing indices?. You can execute steps 3-5 runing SQL commands either at the command line or in phpMyAdmin. Although the post was for drupal, it worked for me on our wordpress instance.