`export default interface` with two names should be an error

The following does not report an error:

export default interface A { a: string; }
export default interface B { b: string; }

and instead makes both names refer to the merged interface.

Sidenote: if this does not become an error for whatever reason, then this should be made to work too:

export default interface A { a: string; }
export default interface B { b: string; }
declare var xa: A;
declare var xb: B;

Start by compiling the two export-default interface declarations shown in the report and inspect the TypeScript compiler's handling of duplicate default interface names. Done means the reported case produces an error, or, if that behavior is rejected, the sidenote declarations preserve distinct A and B references.