If I was to play 2 Sidewinder Slivers would the 'All Slivers have flanking' ability stack causing creatures blocking my Slivers to get -2/-2 until the end of the turn or would it gain flanking twice but the second instance has no effect so creatures blocking my Slivers only get -1/-1 until the end of the turn?
Flanking is a triggered ability, so it can trigger separately for every instance of the ability that the creature has. This is ever so slightly different than just giving -2/-2. When your creature is blocked two abilities go on the stack, each one to give the blocker -1/-1. The end result is indeed that the blocker ends up with -2/-2, but there are some various corner cases you could create with certain cards where it matters!
NB: You can tell it's a triggered ability because it starts with the magic words "When, Whenever, or At."
Each instance of Flanking triggers separately. After the stack resolves, creatures blocking will have -2/-2 until end of turn.
702.24a Flanking is a triggered ability that triggers during the declare blockers step. (See rule 509, "Declare Blockers Step.") "Flanking" means "Whenever this creature becomes blocked by a creature without flanking, the blocking creature gets -1/-1 until end of turn."
702.24b If a creature has multiple instances of flanking, each triggers separately.
Each sliver will have 2 instances of Flanking. Since those are triggered abilities, each instance will trigger, go to the stack, and resolve separately.