3 links have been added on this idea.
  1. Top answer on quora “YouTube announced publicly that they will show you a video that you already watched because it is likely that you will watch it again.” – but no reason why you can’t just tick a box to stop it suggesting the same video time and time again.

  2. Just realised thousands of people already voted for this – google isn’t bothering doing anything about it. Shame, as I don’t want to click the “not interested” button as that may discourage it from suggesting similar videos, just a no repeats button would do – thought they were clever enough to guess folk didn’t like repeats.