Me and Waisz were playing an MTGO and we came across something we thought was pretty goofy. Essence Sliver gives "Whenever a sliver deals damage, its controller gains that much life." There were two Essence Slivers out and they were gaining 2 life for every damage. Since it is an ability added, isn't it only on there once?

It's an effect. People despised my sliver deck because I abused that, as well as other things, oh so badly. I had four of them out quite often.

It is like with muscle slivers. You get +1/+1 for everyone in play.

If it gives all slivers that ability, then it doesn't stack. If the ability is inherent to the card itself, then it would stack, as each one is doing it seperately.

Every time a sliver deals damage, the Essence Sliver's ability triggers. If you have 2 Essence Slivers, each of them triggers when damage is assigned. MTGO is right... it usually is.

The ability stacks, if you have two, say, Winged Slivers out, they technically do have 2 Flying abilities there. Just so happens for the life gain, you notice it. It isn't a keyword, it is a specific set of words added to the text. Two of them add it twice. Same reason two Loxodon Warhammers trigger the life gain twice, they grant the ability to the creature itself, twice.

Post the essence sliver, since the wording on the card makes a big difference.

You can look at my first post, Delphi. ANd reading it now after not being annoyed about it makes more sense now.