Update docs, ensure conflicting flags cannot be combined

README.md

@@ -35,7 +35,7 @@ You can also pass additional flags to `JsonParser::lint/parse` that tweak the fu
 - `JsonParser::ALLOW_DUPLICATE_KEYS` collects duplicate keys. e.g. if you have two `foo` keys they will end up as `foo` and `foo.2`.
 - `JsonParser::PARSE_TO_ASSOC` parses to associative arrays instead of stdClass objects.
 - `JsonParser::ALLOW_COMMENTS` parses while allowing (and ignoring) inline `//` and multiline `/* */` comments in the JSON document.
-- `JsonParser::ALLOW_DUPLICATE_KEYS_TO_ARRAY` collects duplicate keys. e.g. if you have two `foo` keys they will saved as array elements in object property (array key) `__duplicates__`.
+- `JsonParser::ALLOW_DUPLICATE_KEYS_TO_ARRAY` collects duplicate keys. e.g. if you have two `foo` keys the `foo` key will become an object (or array in assoc mode) with all `foo` values accessible as an array in `$result->foo->__duplicates__` (or `$result['foo']['__duplicates__']` in assoc mode).
 
 Example:
 

src/Seld/JsonLint/JsonParser.php

@@ -202,6 +202,10 @@ public function lint($input, $flags = 0)
      */
     public function parse($input, $flags = 0)
     {
+        if (($flags & self::ALLOW_DUPLICATE_KEYS_TO_ARRAY) && ($flags & self::ALLOW_DUPLICATE_KEYS)) {
+            throw new \InvalidArgumentException('Only one of ALLOW_DUPLICATE_KEYS and ALLOW_DUPLICATE_KEYS_TO_ARRAY can be used, you passed in both.');
+        }
+
         $this->failOnBOM($input);
 
         $this->flags = $flags;