The AVERAGE number of shares before a block is solved is equal to the difficulty. It is possible (but VERY unlikely) for a block you are working on to have no solution. However since the block you are working on changes all the time, it is not a problem. Every time a transaction is added to the block or someone else find a block the one you are working on changes.

There is no value to shares but they are a useful thing for pools to track because they can be verified and there is no way to looking for valid shares without looking for a valid block.

There is no duplication of work in mining pools. There is a number in the block that gets changed for every hash, the pool sends out work that start each worker at a different point. This does not give larger pools any noticeable advantage because there are many many possibilities to try and only a tiny fraction are tested before the block changes.

Edit: Also there is no duplication of work between pools or solo miners because the block you are working on contains the address to be paid the block reward.