Fatal error: Cannot redeclare template_preprocess_select() (previously declared in /Users/ron/Sites/provgmc.dev/core/includes/form.inc:30) in /Users/ron/sites/provgmc.dev/core/includes/form.inc on line 37
This is not an issue with code I've written or altered. I'm just trying to install a theme using the user interface. I tried Bootstrap and Zymphonies, then a simple module or two; they give the same error when trying to install them.
I am using Drupal 8.2.6, running on Mac OS X Sierra, with PHP 5.6.28. I am not using OPCache, and there are no changes from Mac OS pre-installed versions.
I've used Drupal 7 (under El Capitan) in the past, and I have never had any issues like this.