Add duplicate memoization prevention

lib/weak.js

@@ -14,6 +14,7 @@ var customError      = require('es5-ext/error/custom')
   , resolveNormalize = require('./resolve-normalize')
 
   , slice = Array.prototype.slice, defineProperties = Object.defineProperties
+  , hasOwnProperty = Object.prototype.hasOwnProperty
   , clearOnDispose;
 
 clearOnDispose = function () {
@@ -26,6 +27,10 @@ module.exports = function (memoize) {
 		var map, length, options = normalizeOpts(arguments[1]), memoized, resolve, normalizer;
 
 		callable(fn);
+
+		// Do not memoize already memoized function
+		if (hasOwnProperty.call(fn, '__memoized__') && !options.force) return fn;
+
 		length = resolveLength(options.length, fn.length, options.async && extensions.async);
 		options.length = length ? length - 1 : 0;
 		map = new WeakMap();