NEW YORK -- The many Bernard Madoff investors who withdrew money from their accounts over the years are now wrestling with an ethical and legal quandary. What they thought were profits was likely money stolen from other clients in what prosecutors are calling the largest Ponzi scheme in history. Now, they are confronting the possibility they may have to pay some of it back.The issue came to the forefront this week as about 8,000 former Madoff clients began to receive letters inviting them to apply for up to $500,000 in aid from the Securities Investor Protection Corp.