I looked on Guild Wars Wiki for some answers and heres what I found.
"A Knight's Insignia is a warrior specific upgrade component that provides reduction of physical damage. This bonus is not global and applies only to the armor piece equipped with the insignia."
This confused me, but sort of gave me the impression that they would stack by saying the bonus is not global.
I looked a bit further and found this.
"A Warrior equipped with a full set of Knight's Insignia (damage reduction -3) and a Rune of Superior Absorption will have a total damage reduction of -6."
This clearly states that having all your armor inserted with the Knight's insignia would not stack and only benefit the wearer by -3 damage reduction.. leaving me a bit confused

Can anyone give me some solid info on how insignias work concerning if they stack or not please. Thanks in advance.