This MetaSO question deals with that:
Asking Nearly Duplicate Questions
Basically the answer is (as mentioned):
- Explain your question clearly and specifically
- Link to the original (similar question)
- Explain what worked/didn't work from that question
- Specifically highlight what you are asking that is different from the original question
That way the last note the Answerers will read is the part that is explicitly different, and they will be able to focus their responses on that, instead of the 'duplicate' part.