Unexpected repeated collisions in SpriteKit - swift

Question

I'm making my first game with Swift and SpriteKit and while I've had collisions working properly for some time now, I recently noticed a huge bug. My collisions can either be the user with and enemy (spaceship against alien) user shooting an enemy (laser against alien). The latter works fine, but recently the collision detection when the ship touches an alien hasn't been working-- normally the alien should be removed from the scene when it touches the so that only a single life (1 of 3 total) is removed. However, the ship looses 3-6 lives upon touch now. Here's the code and where I'd assume the problem is:
This is one of a few contact functions that is then called in the general contact method.
func alien_ship_contact(contact:SKPhysicsContact){
var alien:SKNode? = nil
if contact.bodyA.categoryBitMask == PhysicsCategory.Alien && contact.bodyB.categoryBitMask == PhysicsCategory.Ship{
alien = contact.bodyA.node
}
else if contact.bodyB.categoryBitMask == PhysicsCategory.Alien && contact.bodyA.categoryBitMask == PhysicsCategory.Ship{
alien = contact.bodyB.node
}
else{
return
}
killOffAlien((alien)!)
aliensKilled = aliensKilled + 1
shipLives = shipLives-1
aShip.lives = aShip.lives - 1
print("ship/alien contact")
}
Here is the killOffAlien function:
func killOffAlien(alien:SKNode){
print("Kill off")
//alien.removeFromParent()
func stopMotion(){
alien.physicsBody?.categoryBitMask = 0
alien.physicsBody?.collisionBitMask = 0
alien.physicsBody?.contactTestBitMask = 0
alien.physicsBody?.dynamic = false
alien.physicsBody?.velocity = CGVector(dx:0, dy:0)
alien.removeActionForKey("facialMotion")
}
func removeAlien(){
alien.removeFromParent()
}
let stopMoving = SKAction.runBlock(stopMotion)
let fadeOut = SKAction.fadeOutWithDuration(1)
let removeFromParent = SKAction.runBlock(removeAlien)
let die = SKAction.sequence([stopMoving, fadeOut, removeFromParent])
alien.runAction(die)
}
And here is the general contact method:
func didBeginContact(contact:SKPhysicsContact){
alien_laser_contact(contact)
alien_ship_contact(contact)
....
Any help would be awesome, I would think that upon initial contact the alien has it's bitmask set off from Alien so that in itself would prevent future collisions as every alien should only be able to remove at max one life from the ship.