Information Diffusion

I've got a problem with netlogo: I want to spread an information in a network. One turtle has the information and gives it to its link-neighbors with a constant probability. This is the code i have so far:

to spread
  if (count turtles with [informed? = true] > .7 * count turtles) [stop]
  ask turtles with [ informed? = true ]
  [
      ask link-neighbors
      [
        if (random-float 1 <= 0.02)
          [
            set informed? true
             show-turtle
            set color green
          ]
        ]
      ]

  set num-informed count turtles with [informed? = true]
  tick
end

Now I want to know: How can I ensure, that every turtle gets the information only ONCE and is not informed twice? I tried "if not informed?", but that only got me error messages. And did I get the command "if (random-float 1 <= 0.02)" right, if i want the information to be spread with a constant probability of 2%?

This should work (not tested). Assumes that you have done set informed? FALSE when you set up the turtles.

to spread
  if (count turtles with [informed?] > .7 * count turtles) [stop]
  ask turtles with [ informed? ]
  [ ask link-neighbors with [ not informed? ] ; **<= my change**
    [ if (random-float 1 <= 0.02)
      [ set informed? true
        show-turtle
        set color green
      ]
    ]
  ]

  set num-informed count turtles with [informed?]
  tick
end