[Feature] Generate Warning for Multiple Traps on the Same Signal

[fxierh]

For new checks and feature suggestions

• [x] https://www.shellcheck.net/ (i.e. the latest commit) currently gives no useful warnings about this
• [x] I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

#!/usr/bin/env bash

cleanup() {
    echo "foo"
}

cleanup2() { 
    echo "bar"
}

echo "start"
trap cleanup EXIT
trap cleanup2 EXIT

Here's what shellcheck currently says:

$ shellcheck myscript
No issues detected!

Here's what I wanted or expected to see:

A warning.