Edit in JSFiddle

$(function(){
	var uniquecheck = $('.singlecheck').find('input[type=checkbox]');
	uniquecheck.click(function() {
		var group = $(this).attr('data-scgroup');

		uniquecheck.filter(function() {
			return $(this).attr('data-scgroup') == group;
		}).not(this).removeAttr('checked');

	});
});

<p>List.1</p>
<ul class="singlecheck">
	<li><label class="g1" for="check1"><input type="checkbox" name="check1" id="check1" data-scgroup="group1">list.1</label></li>
	<li><label class="g1" for="check2"><input type="checkbox" name="check2" id="check2" data-scgroup="group1">list.2</label></li>
	<li><label class="g1" for="check3"><input type="checkbox" name="check3" id="check3" data-scgroup="group1">list.3</label></li>
	<li><label class="g1" for="check4"><input type="checkbox" name="check4" id="check4" data-scgroup="group1">list.4</label></li>
</ul>
<p>List.2</p>
<ul class="singlecheck">
	<li><label class="g1" for="check5"><input type="checkbox" name="check5" id="check5" data-scgroup="group1">list.5</label></li>
	<li><label class="g1" for="check6"><input type="checkbox" name="check6" id="check6" data-scgroup="group1">list.6</label></li>
	<li><label class="g2" for="check_A"><input type="checkbox" name="check_A" id="check_A" data-scgroup="group2">list.A-1</label></li>
	<li><label class="g2" for="check_B"><input type="checkbox" name="check_B" id="check_B" data-scgroup="group2">list.A-2</label></li>
	<li><label class="g3" for="check_C"><input type="checkbox" name="check_C" id="check_C" data-scgroup="group3">list.B-1</label></li>
	<li><label class="g3" for="check_D"><input type="checkbox" name="check_D" id="check_D" data-scgroup="group3">list.B-2</label></li>
</ul>