Ah yes now that you mentioned it, I ran a test and yes, it does not stack regardless on how many you land a hit on that unit (1 unit).
But, it stacks per unit.
Let's say you have 3 different units that all of them has Slow Poison ability that slows MS by 5%.
These 3 units attack 1 enemy unit at a time, thus the slow would be 15%.
The stacking type is per unit count.
And let's say you have 1 unit that has Slow Poison ability that slows MS by 10%.
No matter how many hit the unit lands on the enemy unit, the MS slow will remain by 10%, unless another source starts to land a hit on that unit, the MS slow will become 20% (2 sources).