Check Numbers Entered

I am constructing an HTML questionnaire and one of the questions
requires people to rate some choices from 1 to 5, where 1 is their
favourite and 5 is their least favourite:

Car
Bus
Taxi cab
Train
Airplane

Each choice has an INPUT TYPE=TEXT tag to contain the response. I
also have a function that is called ONCHANGE in order to check that a
number is entered between 1 and 5:

function CheckNos(obj) {
if(obj.value.match(/[^d]/) || obj.value<1 || obj.value>5)
{alert("Please enter a number between 1 and 5");obj.value="";return
false}
}

This works fine, but ideally I would like a function that could check
ONCHANGE that someone has not filled in the same number twice, for
example answered "1" for all of them. They are only allowed to use
each rating number once, so rating their choice 1 to 5.

Can anyone suggest a short function that could accomplish this easily?
Could I use some kind of array with 5 elements and allocate a flag
value once that number had been chosen, then check that the value had
been set?