Fatal error: Cannot redeclare when switching themes

The CSS for my parent theme (core) was incorrectly formatted. The code at the top read:

Theme Name: Core
Theme URI: http://www.domain.co.uk
Description:  The core theme.
Author: Squideyes
Author URI: http://www.domain.co.uk
Template: Core
Version: 1.0.0
Tags: clean, simple, easy

The Template: tag should only appear in the child theme css. This is what caused the functions conflict.