I have a field with multiple options and I want to allow it to have only one option selected at the same time but user can hold CTRL key and
I had some dealings with the select \ multi-select this is what did the trick for me
<select name="mySelect" multiple="multiple">
<option>Foo</option>
<option>Bar</option>
<option>Foo Bar</option>
<option>Bar Foo</option>
</select>
Just don't make it a select multiple, but set a size to it, such as:
<select name="user" id="userID" size="3">
<option>John</option>
<option>Paul</option>
<option>Ringo</option>
<option>George</option>
</select>
Working example: https://jsfiddle.net/q2vo8nge/