How to use the path suffix in the Metatag page title?
The only token Pathauto provides is [node:url:args:join-path], the generated alias for the node. The suffix originates from, pathauto_alias_uniquify(&amp;$alias, $source, $langcode), which Pathauto doesn't expose in any way; it's generated as needed to ensure the output alias is unique.

Theoretically, you could provide your a custom token that tries to find the value, but it would be error prone; pathauto isn't storing the suffix independently so there's no guarantee that a matched value is actually the suffix generated from Pathauto. 

This is more of an editorial problem then a technical one. So a better solution would be to enforce unique titles at an authoring level. 

For example, utilizing the Unique Content Title module to verify that each node title is unique upon submission.