Some easy ones (?)

["Scott E. Fahlman" <Fahlman@C.CS.CMU.EDU>]

    If let* doesn't allow duplicate names, then we'll end up writing
    nested let* expressions (which is senseless, let* was invented to
    avoid nested let expressions) or using setq more than necessary.

I can't imagine any reasonable piece of code that would use the same
name twice in a LET*.  At least, I can't imagine any such code that
would not be much clearer if done some other way.  So I don't think that
we'll end up writing nested LET* forms if we outlaw duplicate variable
names in a LET*.  Unless oyu can supply a reasonable example, I think
we're better off not making an exception for this one case.

-- Scott