If you see error message 'PHP Fatal error: Cannot redeclare function' or similar error message in your script, it means there is a problem with your script code (it tries to declare the same function multiple times).
This error says that your function is already defined. This could mean:
- you have the same function defined in two files
- you have the same function defined in two places in the same file
- the file in which your function is defined is included two times (so, it seems the function is defined two times)
To help with the third point, a solution would be to use include_once instead of include when including your functions.php file -- so it cannot be included more than once.