It seems that there is always someone who has already seen, tested, developed, etc anything you are working on.

The real point is that it may be true. They may have really seen something similar in their past experience but why did they put it apart?

Times were not ready, technology was not there, customer wont accept it or it was simply a great mistake?

My point is, you can say pretty much anything on an innovative idea. After you said that just try to add some value or people will think you are make an implicit assertion that you made a mistake in the past.

This lead us to an interesting point that I think is worth a post at some point in time. After you have run your due diligence on an idea, and having proved that it does not fit your current scenario, you should put it somewhere in your innovation process just to make sure that at regular intervals you check it again.