Guild Wars 2

Do sigils stack i just want to be 100% sure. Can i stack 2 undead slaying on my thieves d/d and than do 20% dmg ?

Same name sigils do not stack.

No, same sigils don’t stack.

for what you listed, you want the undead slaying, and sigil of force(albeit only 15% total). you can’t stack 2x the same,

“Static bonuses stack with themselves when used on dual-wielded weapons, except for Superior Sigil of Force (only +5% bonus to damage) , Superior Sigil of Accuracy (only +5% bonus to crit chance) and Superior Sigil of the Night (only +10% bonus to damage). However Superior Sigil of Force and Superior Sigil of the Night do stack to a total of 15% bonus to damage during the night.”

So the sigil of undead slaying should stack just fine for the +20% damage.

Source: http://wiki.guildwars2.com/wiki/Sigil

On kill sigils also do not stack even if they r different types.