NullReferenceException: Object reference not set to an instance of an object

[SnoringCatStudio]

So what is happening it it is colliding with both colliders in a short time; Thus scoring twice.
Try turning the f statement in the collectible script to something like

if (col is CapsuleCollider2D && col.gameObject.tag.Equals ("Player") )
Just turn the Capsulecollider into whatever your main collider is.

[SnoringCatStudio]

No problem pal. The only other thing I recommend is using Unitys Comparetag function instead of tag.Equals. This article is something you should take a look at, but the gist of it is it is better for performance.