Like I said previously, you're offering them 10% more because of market conditions(you need some fresh meat on your project ASAP), not because of their (yet unknown) performance, and hedge your bets that the ones already working for you won't find out(here salaries are confidential) or are to tied down to be looking for new jobs so soon.

I hate this approach, but for most companies in Europe it's the reality that works in their favor.