Buffs of the same type will NOT stack, but there are some similar abilities that do work together. For example, the attack buff from Ares, Boldtusk, and Kiril do not stack - so whichever one you cast LAST takes priority. However, Wu Kong will stack with those attack buffs because he also reduces accuracy, whereas Brienne will not stack because her ability is just an attack buff though it changes as you get hit. Similarly, defense dubuffs such as Grimm, Gormek, Tiburtus, Isarnia, and Athena do not stack, even though Athena’s is slightly different again. Another notable exception is Alberich, who does stack with the mana regen from Lancelot and Khagan because their mechanics are slightly different.
This also works in reverse, where an attack buff will override an attack debuff, and vice versa. i.e. if you get hit with Skittleskull’s special and then cast Boldtusk’s special, you will cancel out the attack reduction and also gain Boldtusk’s attack increase in full. This is a very useful interaction to note for titans, where bear banners and turtle banners will reverse the effect of green and blue titans respectively.