How to fix Fatal error: Cannot redeclare get_cli_args() in class-wp-importer.php

Are you somehow including wp-admin\includes\class-wp-importer.php using include or require? That may cause the error of get_cli_args() function declared twice.

If that is the case, then you better use include_once or require_once.

Also, perhaps you should test your installation by disabling all the plugins and any custom theme. Then activate the theme and test WordPress. Then activate one plugin at a time and test each time to see if the error appears. This will show if this error is caused by any plugin or theme.