According to Markovnikov rule and stability of carbocation which is most likely to form if HI "excess" is not given?
There are three reasons as far as I can see as to why the 1,2 product is favored. There may be more.

Steric hindrance
The iodine atom is large in size compared to the carbon atom. Having two iodine atoms on successive carbon atoms in a hydrocarbon is quite unstable. In fact, should two successive carbon atoms possess iodine substituents, they usually undergo elimination reactions. And here the 1,1 product would have two iodine groups on the same carbon atom!

Inductive effect of halogens
While it is true that halogen atoms have a positive mesomeric effect, this effect is almost always dominated by their negative inductive effect. If you see a halogen atom, you can usually neglect it's +M effects in favor of it's −I effects.

Carbocation stability
The carbocation formed in the process of formation of the 1,2 product is more stable (due to hyperconjugation) than the one formed in the process of formation of the 1,1 product (again, due to the −I effect of iodine).


Either way,  1,2-diiodopropane is more stable than  1,1-diiodopropane.