Warn when adding a workflow input with the same name as a previously added workflow input

[jvdzwaan]

This happens when copy/pasting wf.add_input() and you forget to change the name.

This does not lead to a validation error (if the types are the same), and causes cwltool to use the second workflow input instead of the first. This leads to unexpected results.

[jvdzwaan]

Actually, it is probably better if an error is raised (instead of a warning).