Warn when duplicate arguments are found

Currently, duplicate arguments result in a categorization but not a
warning, and it's often difficult to find where in the template hierarchy
the problem lies. This causes a warning to be provided containing the
calling page's name, the called template's name, and the parameter's name.

Bug: T85352
Change-Id: I26b9a7ed5a2f246d00a49a5f6effe40b4443a9d0