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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 the creature type of a Sliver changes 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.

The controller of the Sliver that enters the battlefield draws a card, not necessarily the controller of Dormant Sliver.

The last ability Dormant Sliver grants itself triggers when it enters the battlefield.