lib, test: add duplicate symbol checking in E()

Add duplicate symbol checking in E() to avoid potential confusing
result. Increase coverage of internal/errors.

PR-URL: https://github.com/nodejs/node/pull/11829
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

lib/internal/errors.js

@@ -60,6 +60,8 @@ function message(key, args) {
 // Utility function for registering the error codes. Only used here. Exported
 // *only* to allow for testing.
 function E(sym, val) {
+  const assert = lazyAssert();
+  assert(messages.has(sym) === false, `Error symbol: ${sym} was already used.`);
   messages.set(sym, typeof val === 'function' ? val : String(val));
 }