Fatal error: Cannot redeclare blah_function() in ../modules/blah.module

Usually seen on the admin/modules page, the admin/themes page, or after modifying module or theme files.

As the error say, you have duplicate function declaration.
Most common cause seems to be :

On the modules page and themes page, EVERY file in themes/*/*.module (themes/*/*.theme) is evaluated once each.

If you (or someone) tried making their own module by copying an existing directory (which is the correct step 1) but failed to rename every function inside the files to their new theme name (which is the neccessary step 2) you'll get this error.

Go in and physically remove/move the module/theme files you don't use out of the modules/themes directory.

Try to "find in files" for that funcname listed in the errors and see where the dupe is.
Even just having a backup called "Copy of troublesome.module" Is illegal, as the dir scan returns it as another available theme. It gets evaluated, then conflicts with the original.
Rename it to troublesome.module.bak instead

This error can also arise if two different module installs choose to use and bundle the same third party library. This is pretty rare. Do the find-in-files to identify the problem, and enquire with the respective module projects.

PS - NEVER have a module the same name as a theme!