Possible Cause:
Because, EVERY file in themes/*/*.module and themes/*/*.theme is evaluated once, if a module is created by copying an existing module, and the module developer did not rename every function inside the module directory structure, you could have two functions (albeit in separate locations) with the same name.
The first function will be evaluated, and the second will throw an error.

Fix:

1. Do a global search for the function name thats triggering the error.
2. Rename or remove the duplicate function

Note: All files of the form *.module will be evaluated. Utilize an alternate extension like *.module.bak to prevent drupal from scanning the old file.