How can I get greater coverage using Alarm and can it be cast more than once?

Alarm description says

Choose a door, a window, or an area within range that is no larger than a 20-foot cube… A mental alarm alerts you with a ping in your mind if you are within 1 mile of the warded area. (PHB 211)

If you can be over a mile away, a 20-foot cube is not a very large area. And if you are sleeping, a 20 foot cube only gives you 10 feet max on all sides (which isn’t enough warning if you are being attack by someone with 30ft movement speed, for example).

  • Can a Ranger or Druid cast this multiple times, or does casting a second alarm, cancel the first?
  • Is there any precedence for multiple casts of other, similar spells that could be applied?
  • Can a higher level NPC or PC increase the area of effect?