wordpress child theme loads child style twice, no parent style

Solution:

Make sure the header of style.css theme is a CSS comment (it should start with /* and end with */) and it has this line:

/* 
Template:     twentyfifteen
*/

Rule of thumb in programming:

Don’t delete code you do not fully understand. Sooner or later it will hit the fan.

More details on child themes and their headers here.

The comment header lines in style.css are required for WordPress to be able to identify the Theme and display it in the Administration Panel under Design > Themes as an available Theme option along with any other installed Themes.