[Spec] Duplicate ambient external module name

[mhegazy]

declare module "fs" {
    var x: string;
}
declare module "fs" {
    var y: number;
}

Expected:
!!! Duplicate identifier ''fs''.
!!! Ambient external module declaration cannot be reopened.
Actual: no error

[ahejlsberg]

Changing to spec issue. The new compiler supports merging of all module declarations, including ambient external module declarations. I see no reason why we should only allow merging of internal modules and specifically disallow merging of ambient external modules. In particular, declaration files sometimes extend existing internal modules and may want to do exactly that for ambient external modules.

[sophiajt]

From design meeting: agreed this should merge just like internal modules.

[sophiajt]

Leaving open so we can make the text change to the spec

[iislucas]

When I run this code in typescript I get

ldixon/foo.ts(4,16): error TS2000: Duplicate identifier '"fs"'. Additional locations:
    ldixon/foo.ts(1,16)
ldixon/foo.ts(4,16): error TS2191: Ambient external module declaration cannot be reopened.

Sounds like the correct behaviour is no errors. Would be great to have an npm publish of a version with this fixed :)

[basarat]

agreed this should merge just like internal modules.

DT actually needs this. Thanks guys (/cc @Bartvds @vvakame will possibly make non-instantiated modules less necessary)