I have a couple of Trepanation Blades, and I was wondering if the effect would stack if I have multiple equipped to one creature.
Yes. When the equipped creature attacks, both triggers go on the stack at the same time (in an order of your choice), then are resolved one at a time.
So, for example, I attack with an Invisible Stalker wielding two Trepanation Blades. Both triggers go on the stack. We resolve the first: spell, spell, land; the Stalker gets +3/+0 until end of turn. We resolve the second: spell, land; the Stalker gets +2/+0 until end of turn. Now my Stalker is a 6/1.
Everyone so far is correct, I am only entering another answer to paste in the rules, because they wouldn't fit in a comment.
Basically since it is a triggered ability, as AndSoYouCode points out, they just all get addded to the stack and then the stack empties doing each effect one at a time.
Nothing is specifically mentioned about duplicate cards, just check for everything that triggers on the event and add it to the stack in APNAP order.
603.3. Once an ability has triggered, its controller puts it on the stack as an object that’s not a card the next time a player would receive priority. See rule 116, “Timing and Priority.” The ability becomes the topmost object on the stack. It has the text of the ability that created it, and no other characteristics. It remains on the stack until it’s countered, it resolves, a rule causes it to be removed from the stack, or an effect moves it elsewhere.
603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities he or she controls on the stack in any order he or she chooses. (See rule 101.4.) Then the game once again checks for and resolves state-based actions until none are performed, then abilities that triggered during this process go on the stack. This process repeats until no new state-based actions are performed and no abilities trigger. Then the appropriate player gets priority.
Yes they do. You might confuse this triggered ability with a replacement effect. Replacement effects doesn't stack because, as the name suggests, they replace the effect.
A quick googling gave me this: http://forums.mtgsalvation.com/showthread.php?t=317005