You are completely ignoring the fact that if an employee is able to come up with a significantly higher offer every 6-12 months, then the company is probably paying him way below the market rate, otherwise it won't be so easy to come up with a competing offer every 6-12 months.

So if an employee you have hired is able to get a competing offer every 6-12 months, it is worth spending time to figure why things are so screwed up in the company and resolve that.