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Sliver creatures you control have "Whenever this creature attacks, it deals 1 damage to any target."
Abilities that Slivers grant, as well as power/toughness boosts, are cumulative. However, for some abilities, like flying, having more than one instance of the ability doesn’t provide any additional benefit.
If you change the creature type of a Sliver you control so it’s no longer a Sliver, it will no longer be affected by its own ability. Its ability will continue to affect other Sliver creatures you control.
If you control multiple Thorncaster Slivers and a Sliver you control attacks, each ability will trigger separately.
