More than one function was declared with the #[main] attribute.

Erroneous code example:

#![feature(main)]

#[main]
fn foo() {}

#[main]
fn f() {} // error: multiple functions with a `#[main]` attribute

This error indicates that the compiler found multiple functions with the #[main] attribute. This is an error because there must be a unique entry point into a Rust program. Example:

#![feature(main)]

#[main]
fn f() {} // ok!