It appears the buffs to the player granted by Spriggans do not stack, and I cannot see why, beyond this being a bug. Am I missing some mechanic here?
Note: Easily tested, with 4 points in Spriggan’s Aura of Retribution. Note your crit chance. Summon first Spriggan and your crit chance will increase by +8%. Transform into Spriggan Form yourself to summon the 2nd Spriggan. Your crit chance will not increase further. I assume the same will be the case with its Spell Damage buff too.