Duplicating a subcommand name as a subcommand alias is not prevented

[epage]

The following should panic in a debug assert but doesn't. It should mirror the asserts for duplicate flags

#!/usr/bin/env -S rust-script --debug

//! ```cargo
//! [dependencies]
//! clap = { path = "../clap", features = ["env", "derive"] }
//! ```

fn main() {
    let mut cmd = clap::Command::new("myprog")
        .subcommand(clap::Command::new("foo"))
        .subcommand(clap::Command::new("bar").alias("foo"));
    cmd.build();
}