It will. But it is a matter of “how much more benefit will i get versus putting something else there?”
Stacking penalties for modules applies thusly:
First module that affects a specific stat (usually the one that affords the largest bonus): 100% of that module’s bonus is applied.
Second module that affects a specific stat: ~87% of that module’s bonus is applied.
Third module that affects a specific stat: ~50% of that module’s bonus is applied.
Fourth modules that affects a specific stat: ~20% of that module’s bonus is applied.
Notice a pattern?
The effectiveness of a module’s bonus drastically decreases after the second or third module (that affects the same specific stat).
This is why you may see 2 or even 3 modules that affect a specific stat, but no more (example: shield or armor resistance modules).
In your particular case; the first Mag Stab will afford all of its bonuses without penalty. The second one will give about 87% of its bonuses.
That makes it still worthwhile to fit.
However, everything ultimately comes down to taste. You are more than welcome to fit more tanking modules you prefer more defense. Then again… since your skills are still relatively low, having that extra damage could mean the difference between clearing out a room more quickly or breaking the tank of a particularly powerful target.
