[cfe] No error if extension type implements the same type twice

[sgrekhov]

Extension types specification was recently updated and now contains

It is a compile-time error if two elements in the type list of the implements clause of an extension type declaration specifies the same type.

There is no this error in cfe

extension type ET(int _) implements int, int {} // no error

Related analyzer issue #53790

Tested on Dart SDK version: 3.3.0-edge.b2a158cc431a1fb0e96c3038e5441fa4de9205e7 (be) (Wed Oct 18 06:59:38 2023 +0000) on "linux_x64"