Why do cameras have double lenses? I was wonder why cameras the good ones have two lenses instead of one?
what benfit does it carry with this fact?
I have told that old cellular phones have camera with one lens and hence it blurring the 
picture, why is that? and why 2 or more lenses are solving this or other problems?
Has I know every configuration of lenses are equivalent to "one lens" isn't it?
 A: Sometimes lenses are paired to form an achromatic doublet.  This is done to reduce the effects of chromatic aberration, which results in blurring because different light colors end up being focused at different points due to the dependence of the refraction process on the frequency of the light.
A: A Double Gauss has two lens groups and 7 lenses.  These all help deal with optimum optical performance of comercial cameras while minimising aberrations.
From the link:

an apt combination of aperture, field, and design complexity.

Lenses in mobile phones are typically aspheric plastic lenses that do well for the conditions they are expected to be used in but obviously suffer in comparison to large scale SLR type lenses.
The reasons for combining lenses (actually it is more 'optical surfaces') is as twistor gave in his answer.
A: First, we need to clear up the terminology a bit. What most photographers and laypeople refer to as "lenses", would be called "lens systems" or "objectives" in a more technical context. As others have pointed out, these consist of multiple simple lenses. They are designed to correct aberrations and have other features like zoom and focus.
But as you mention camera phones, I suspect you are referring to dual cameras like on the iPhone, which has two independent lens systems. The idea here is that you have a camera with a wide view, and a camera with a telephoto view. Then, just as a photographer might carry around 2 lenses (or "objectives") for their DSLR and switch them around depending on the photo they want to take, an iPhone user can decide whether to use the wide angle or telephoto lens.
Other manufacturers make different claims about how it improves the image using software, but that depends on the implementation.
A: I will assume that you want to get the answer to why cameras have an objective-eyepiece combo instead of a simple lens, that is with the combined power of both.
I will start with a single lens system, where you have a lens with fixed focal length.

Note here that the light sensitive film needs to be placed at a point where light rays converge together. So, when you want to take an image at infinity, you would keep the film at focus of lens. In other cases at other places.

That effectively means the distance need to be changed between the lens and the CCD, the light sensitive part in a digital camera. This can pose an engineering problem, since this adjustment of focus can be very delicate and a small deviation can hamper the quality of photo considerably.
Now let's move on to a 2 lens system. Here, what you have is a moveable objective lens and a stationary eye piece towards the CCD sensors. So now the previous problem is eliminated.
So, now one has to only change the distance between objective and eye piece to get images at any distance. This is the movement what you see when you actually zoom the camera, lenses come out and try to zoom onto the image.
In modern cameras the lens system consists to half a dozen or so lenses. As @Nic pointed out a double Gauss system contains 7 lenses. This is to further reduce the imaging defects like chromatic aberration and coma formation.
