Make "allow option overwriting" configurable

[remkop]

Currently picocli accepts multiple occurrences of options with arity=1 and uses the last value.
It may be desirable to throw a SuperfluousParameterException instead.

@ddimtirov suggested making validation behaviour configurable:

1. accept and silently use last value
2. accept and emit warning while using last value
3. reject (throw exception)

[ddimtirov]

Also "warn" if parameters values diverge may be a good idea.

…

On Thu, May 25, 2017, 02:26 Remko Popma ***@***.***> wrote: Currently picocli accepts multiple occurrences of options with arity=1 and uses the last value. It may be desirable to throw a SuperfluousParameterException instead. @ddimtirov <https://github.com/ddimtirov> suggested making validation behaviour configurable: 1. accept and silently use last value 2. accept and emit warning while using last value 3. reject (throw exception) — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#129>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAV8wqeXoGXgMksxtETp0dz_HNUDros9ks5r9Gg9gaJpZM4NlbOY> .

[remkop]

#138 has been implemented, with an option to switch the validation off (the last specified value is used in that case). What hasn't been done yet is providing an option to emit warnings

[remkop]

Created #158 for the remaining work. Closing this ticket.