Operate on the cross product of a list, but avoid duplicates

# Don't operate on B and A when A and B have already been done.
proc foo data {
    while {[llength $data]} {
	set data2 [lassign $data value1]
	foreach value2 $data2 {
	    # doSomething $value1 $value2
	}
	set data $data2
    }
}