14.1.1 Static Semantics: Early Errors,
FormalParameters : FormalParameterList
> It is a Syntax Error if the source code matching this production is strict code and BoundNames of FormalParameterList contains any duplicate elements.
But for strict code the early error restriction from StrictFormalParameters is already applied, cf. 14.1.1 and 14.4.1:
> If the source code matching this production is strict code, the Early Error rules for StrictFormalParameters : FormalParameters are applied.
That means the duplicate parameters check can be removed from "FormalParameters : FormalParameterList".
I don't think so. StrictFormalParameters isn't specified as being automatically substituted for FormalFarameters in strict code. Instead, it only applies where it is explicitly used in grammar production. It still seems like it need to be in both places to cover those two situations.
Sorry, I was wrong, there is the general statement about StringFormalParameters so you are correct
fixed in rev21 editor's draft
fixed in rev21 draft