The US has tax treaties with most countries to avoid dual taxation. You still have to file a return, but except in rare circumstances, you will never have to pay double taxes to both host country and home country.
But it is a ball ache, not only for the ex pat but also for the banks that deal with them, so much so that there are many reports of banks refusing US citizens accounts and closing ones they do have. There may be no double taxation occurring but there is administration involved in the part of everyone involved. Essentially a ridiculous cost forced on all parties by the US govt.
