HTTP Parameter Pollution
HTTP Parameter Pollution or HPP in short is a vulnerability that occurs due to passing of multiple parameters having same name.
There is no RFC standard on what should be done when passed multiple parameters.
This vulnerability was first discovered in 2009.
HPP could be used for cross channel pollution, bypassing CSRF protection and WAF input validation checks.
Go to https://en.wikipedia.org/wiki/HTTP_parameter_pollution for backend behaviour matrix