You want to update checkUniqueOption() this function and because of that function is private you need to rename this as _checkUniqueOption() or something else, and in that file there are some other functions which are also private so you need to do same thing for that all functions as well. So your new Controller file's content will be :
same issue not override that file,
I have follow same as per above but not working
need to override this : Magento\Catalog\Model\Layer\FilterList
<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
You can create new module and override that file otherwise you do that using custom theme as well.
I'll show you here how can we override template file using custom module.
Please create Vendor name directory first, and move into that and create another directory Module.
Then you need to create registration.php file here.