title: Collision between two nodes happened twice at the same time
body: <p>In ARkit, I have two nodes such as bullet node and target node,</p>
  <p>bullet node is below</p>
  <pre><code>  let body = SCNPhysicsBody(type: .dynamic, shape: SCNPhysicsShape(node: bullet,  options: nil))
          
          body.isAffectedByGravity = false
          bullet.physicsBody = body
          bullet.physicsBody?.applyForce(SCNVector3(orientation.x*power, orientation.y*power, orientation.z*power), asImpulse: true)
          bullet.physicsBody?.categoryBitMask = BitMaskCategory.bullet.rawValue
          bullet.physicsBody?.contactTestBitMask = BitMaskCategory.target.rawValue
          bullet.physicsBody?.collisionBitMask = BitMaskCategory.target.rawValue
  </code></pre>
  <p>Target node is below</p>
  <pre><code>let boundingSphere = targetNode!.boundingSphere;
          let radius = boundingSphere.radius;
          let center = boundingSphere.center;
          let sphereShape = SCNSphere(radius: CGFloat(radius));
          var shape = SCNPhysicsShape(geometry: sphereShape, options: [SCNPhysicsShape.Option.scale : targetNodeScale * 1.1])
          targetNode?.physicsBody = SCNPhysicsBody(type: .dynamic,shape: shape)
          targetNode?.physicsBody?.isAffectedByGravity = false
          targetNode?.physicsBody?.categoryBitMask = BitMaskCategory.target.rawValue
          targetNode?.physicsBody?.contactTestBitMask = BitMaskCategory.bullet.rawValue | BitMaskCategory.target.rawValue
          targetNode?.physicsBody?.collisionBitMask = BitMaskCategory.target.rawValue | BitMaskCategory.bullet.rawValue
          targetNode?.setValue(0, forKey: &quot;hitCount&quot;)
  </code></pre>
  <p>And I found when these two nodes hit each other, the below method was invoked twice almost at the same time consecutively but it should only happen once.</p>
  <pre><code>func physicsWorld(_ world: SCNPhysicsWorld, didBegin contact: SCNPhysicsContact) {
  
  }
  </code></pre>
  <p>And sometimes it happened, some times didn't, so I assume might be there are some collision precision should be added, does anyone know how to do that part? Thanks</p>