Avoid confusion of cards: Do not allow the same title multiple times

[frederikheld]

I guess it should not be allowed to use the same title multiple times.

Imagine two cards with the same title. As long as each of them stays in their Step, you can distinguish them from each others. But usm.io encourages to move cards around and as soon as you put them in different steps, you don't know which is which anymore.

For the deep links feature this would lead to the technical problem, that the running number will most probably be added following the order of the cards in usm.json. If you move them around, the order might change and with it the deep-link will switch between cards :-P

I guess the technical issue could be solved somehow, but it would still be confusing for the user to allow the same title more than once.

Originally posted by @frederikheld in #10 (comment)