Top level checks are only used for miniscript APIs and not descriptor APIs.

    #[test]
    fn test_2() {
        let desc = Descriptor::<String>::from_str("wsh(thresh(1,pk(A),s:pk(B)))").unwrap();
        let desc = Descriptor::<String>::from_str("wsh(thresh(1,pk(A),s:pk(A)))").unwrap(); // This should also ERR because duplicate keys.
        
        // But miniscript APIs are fine.
        let ms = Miniscript::<String, Segwitv0>::from_str("thresh(1,pk(A),s:pk(B))").unwrap();
        let ms = Miniscript::<String, Segwitv0>::from_str("thresh(1,pk(A),s:pk(A))").expect_err("This is not a valid miniscript");
    }

https://github.com/brunoerg/bitcoinfuzz/issues/73#issuecomment-2566020224

We should have those in descriptor APIs too. In the above example, we have duplicate keys being allowed in descriptors.

Start by locating the descriptor parsing APIs and comparing their top-level checks with the existing Miniscript checks shown in the issue. Add a regression test for the duplicate-key descriptor example, and consider the work done when descriptors reject duplicate keys while the existing Miniscript behavior remains covered.