radio button validation

I have an mysql array with several radio buttons,

<INPUT name=”bookout” id=”bookout” type=”radio” class=”formslova” value=’value1′>
<INPUT name=”bookout” id=”bookout” type=”radio” class=”formslova” value=’value2′>
<INPUT name=”bookout” id=”bookout” type=”radio” class=”formslova” value=’value3′>

I want to make a button validation:

if (document.getElementById(“bookout”).checked==false) {
alert(‘error’);

For some reason when I have more that one button in the array only the first counts, meaning only when first is clicked is the button validation checked=true.

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