Spined Sliver

Spined Sliver rulings:

2013-07-01: 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.

2013-07-01: 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.

2004-10-04: If two Spined Slivers are on the battlefield then each blocked sliver gets +2/+2 for each creature blocking it.