Your child theme’s functions.php file should be an empty file to which you can add code specific to your child theme. It appears you have copied the functions.php file from the parent theme instead.
See this note about functions.php in Child Themes:
Unlike style.css, the functions.php of a child theme does not override its counterpart from the parent. Instead, it is loaded in addition to the parent’s functions.php. (Specifically, it is loaded right before the parent’s file.)