Prevent more than one space between words

I have a function that prevents people putting numbers or any symbol but letters onkeypress into a text box due to ongoing problems with data entry.

<td><input type="text" name="name" onkeypress="return isAlfa(event)"></td>

Now some staff for reasons unknown put two spaces between words at random times. So I need to prevent them putting more than one space between words. I want to do this in the same function, but it keeps breaking.

function isAlfa(evt) {
  evt = (evt || window.event);
  var charCode = (evt.which || evt.keyCode);

  if ((charCode > 32)
    && (charCode < 65 || charCode > 90)
    && (charCode < 97 || charCode > 122)
  ) {
    return false;
  }
  return true;
}

How can I prevent them entering more than one space between words?

may be try to use something like this. on your keypress event try to check the last value by using string substr method and if that gives you a space and current code is also a Space then prevent or do whatever you want to do with input.

function checkstuff(event){
  if (event.target.value.substr(-1) === ' ' && event.code === 'Space') {
    console.log('space pressed in sequence');
  }
}

<input type='text' onkeypress="checkstuff(event)">