once: add debug

src/once.js

@@ -2,10 +2,15 @@
 
 exports = module.exports = once;
 
+const debug = require('debug')('box:once');
+
 // https://github.com/isaacs/once/blob/main/LICENSE (ISC)
 function once (fn) {
     const f = function () {
-        if (f.called) return f.value;
+        if (f.called) {
+            debug(`${f.name} was already called, returning previous return value`);
+            return f.value;
+        }
         f.called = true;
         return f.value = fn.apply(this, arguments);
     };