Q:

Duplicates while I did the search

Stack Overflow (SO) is a good source to find answers, as is the rest of the internet. So I'd search on google and when a SO page pops up I always check those question and answers.
When I will ask a question on SO I rephrase my question several times to check for duplicates. I'd like to spend time into asking a question so SO will not be cluttered with all the same questions. However, never I have succeeded.
Clearly I must be doing something wrong:
Is anyone willing to spell out how to use this site so I minimise the duplicate question and thus I can motivate beforehand why it won't be a duplicate, or I won't post the question as it is already answered?
This would save up your and my time.

A:

When I will ask a question on SO I rephrase my question several times to check for duplicates. I'd like to spend time into asking a question so SO will not be cluttered with all the same questions. 

That's good. This is the type of effort that we expect from askers, and you are already doing that. So thank you.

However, never I have succeeded. Clearly I must be doing something wrong.

I'm sorry to hear that. In some ways, I find that hard to believe. You mean to say that you've never managed to find an existing question on a subject, yet it's later been proven to you that there is a duplicate question because someone else found and proposed it?
While it's true that sometimes questions are hard to find for anyone but tag experts (both because they know the perfect search terms and, more likely, because they remember seeing and/or answering that question before), it shouldn't be so bad that someone putting in diligent effort can't find them.
Still, if that's true, then you have nothing to feel bad about. You are trying, and you obviously respect our time enough to search first. This is really all that we can ask of you, or anyone else. There is no magic.
There are almost certainly strategies you could learn and use to improve your search skills, but making recommendations along those lines are beyond the scope of this site.
About the only thing I can tell you is to avoid the search function built into Stack Overflow. It's terrible. Everyone uses Google site search (site:stackoverflow.com) to find questions here. The only time I ever use the Stack Overflow search is when I want to use some of its advanced operators, but that's pretty rare, and not especially useful when your goal is just to find duplicates.

This would save up your and my time.

In fact, you may be doing us all a favor by asking these questions. Whatever do I mean? Well, if it's true what you say—that you are reasonably trying to find existing questions and failing—then obviously the existing questions are not easy enough to find. They either use obscure keywords, have bad titles, are improperly tagged, or something is wrong with them.
If your questions are reasonable and well-asked, then it isn't a problem for someone who knows of an existing question to mark it as a duplicate. Then, your question becomes a useful redirect to the other question. An answer that was previously hard to find because the question that contained it was so obscure now becomes easier to find.
As Joel Spolsky writes here on the blog:

The more chance that someone types a question into Google and finds their exact question already answered, the better a job we’ve done.

In other words, duplicate questions themselves are not bad. Only two things are bad: (1) low-effort questions, and (2) scattering the information about across multiple questions. You are doing your part to prevent #1, so as long as the community does their part to avoid #2, we'll all be okay.

