If I wear contact lens and spectacles of same power, is it same wearing thick spectacles of twice the lens power? Assume I have a contact lens of power -1 and a spectacles of +1.
Will they cancel out their effect? is it same as my normal eye sight?
 A: Approximately the answer is yes. The relevant theory here is Gullstrand's equation. Look this up on the "Gullstrand's Equation" entry on the HyperPhysics site and also see the "Gullstrand's Equation Calculations" page in the same site which will give you the definitions of the symbols.
Approximately, by Gullstrand's equation, the powers add for lens elements near to one another. Powers are the reciprocals of the focal lengths, which is what the +1 and -1 mean on your lenses: these are the powers in inverse meters ($m^{-1}$). So +1 eyeglasses with -1 contacts (or contrariwise) will approximately null each other. As Gullstrand's equations says, there is a correction term for nonzero distances between the lenses, but it will be here very small: if there are, say, 2cm between the two lens elements, the total power in inverse meters is $P_1+P_2-P_1\,P_2\,d\approx -1+1+1\times 0.02$, so you get a nett lens with a power of 0.02.
If you're curious, you should be able to pick up some cheap eyeglasses of the opposite signed power to try the experiment yourself. A better version of the experiment would be to put two opposite power sets of eyeglasses, one behind the other, so that you can put the pair into your ken and out again very quickly to see the effect. Otherwise, the lens of your eye will adjust to the presence of the contact lens and, even if the eyeglass cancels its effect, you'll still see the eyeglass's cancellation of the effect. You can't get your contact lenses in and out quickly enough to avoid the unconscious adjustment that your brain will make. You need to be able to apply the cancelling lenses quickly before eye's lens has time to compensate.
