As Galactic Cowboy mentioned, you should reference the previous question and explain why yours is not a duplicate.
But, you should also be phrasing your question to make it obvious that it is not a duplicate.
Consider the following questions.
- Create Email Application in .NET
- Create Email Application in .NET without using WCF
The root question is obviously the same, but even at the main page, it is immediately apparent why you asked your question instead of simply being happy with the previous one.
After crafting a title which highlights the differences, you should write out your question and include references to the helpful answers in the other questions (the ones that got you almost all the way to your answer) and ask a direct question about only the part that was omitted in the other question.
Try your best not to have that much overlap in your questions if possible. Explicitly mention the areas which overlap and you DO NOT REQUIRE an answer to. That way, no one can accuse you of having a duplicate question, because they are related rather than identical.