Let should not be allowed as identifier in let and const declaration

[yuit]

In ES6 spec in section in Let and Const Declaration

LexicalDeclaration : LetOrConst BindingList ;

• It is a Syntax Error if the BoundNames of BindingList contains "let".
• It is a Syntax Error if the BoundNames of BindingList contains any duplicate entries.

currently we allow

let let = 10;
const let = 30;

which should be syntax error regardless of strict-mode.
Note: below shouldn't be a syntax error unless it is in strict-mode

let = 10
var let = 20;

[DanielRosenwasser]

Duplicate of #3475.