I had the same issue, POS doesn't support variables of the same name via a form POST or GET (even though this is a web standard). To get around it in javascript I loop the mutliselect elements and generate a hidden input and then pass that along with the form

Edit: Sorry I just realized I might not have answered your question, yes standard multiselect forms don't display on the backend correctly but if you name your select dropdown (for example) as name="something[]" (note the square brackets [ ] ) the data will be transfered as an array with all the options.

As Rich correctly pointed out, you need to send an array of values. It is pretty common practice to use [] for arrays and hashes notations.

Array

<input type="textbox" name="cars[honda][]", value="CRX">
<input type="textbox" name="cars[honda][]", value="S2000">
<input type="textbox" name="cars[honda][]", value="NSX">

Hash

<input type="textbox" name="cars[honda][model_name]", value="CRX">
<input type="textbox" name="cars[honda][engine_size]", value="1600">
<input type="textbox" name="cars[honda][seats]", value="4">