Creating a new tag with a value equal to the id of an existing option causes both the new tag and the existing option to be added as selections

// input: red,
// var_dump($_POST["colors"]);
Array
(
    [0] => red
    [1] => red
)

// input: apple,green,water,
// var_dump($_POST["colors"]);
Array
(
    [0] => green
    [1] => green
)

// input: red,green,blue,yellow,
// var_dump($_POST["colors"]);
Array
(
    [0] => red
    [1] => green
    [2] => blue
    [3] => yellow
    [4] => red
)

<select name="colors[]" id="tags-example" multiple="multiple">
    <option value="red">apple</option>
    <option value="green">grass</option>
    <option value="blue">water</option>
    <option value="yellow">sun</option>
</select>

$('#tags-example').select2({
    tags: true,
    tokenSeparators: [',', ' '],
    maximumSelectionLength: 2
});

Please see the following fiddle to see what happens:
http://jsfiddle.net/o1je0988/

Start with the linked jsFiddle and reproduce the issue using the #tags-example select initialized with tags, tokenSeparators, and maximumSelectionLength. Trace the tag and option selection handling, then verify that entering an existing option value creates only one selection in each reported input case.