Parser: check behaviour with duplicated numbered/named parameters

[kynikos]

In a template, it's possible to define a parameter multiple times (and only the last definition is used): the first time the parameter can be positional, but then it can be overridden by a numbered definition, or even a named one, but this case will hardly be solvable since the source of each template should be checked.

Finding duplicated parameters could be better left to be dealt with at the plugin level, to allow adapting to the various specific templates.