At least in non segmented animals like vertebrates, duplication of limbs is more or less out of the question. First of all, a single chance mutation has to create a pair of limbs which just happen to not be in such a place as to interfere with any other organs or anatomical structures. Then, that mutation has to be somehow so advantageous that it takes over the population. That's not going to happen, realistically.
So, mariliths will have to be non-tetrapods. They could be from a parallel lineage of land vertebrates, but I have a different suggestion; giant insects.

Above is the larva of a water beetle. It has six limbs and a serpentine abdomen - just like a marilith. Suppose that, in an oxygen rich world, a neotenic form of water beetle evolves giant size. It would be an ambush predator, with binocular vision - hence a vaguely humanoid face - and stuff huge raptorial limbs. It would lie in wait on the water's edge, and, when a prey item came near, rear up in a pose much like the typical naga posture.