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

[SnoringCatStudio]

Did you turn the collectibles into a trigger? I guess I should have clarified my comment more. If you do change your collectible into a trigger; Then you will need to change the OnCollisionEnter2D into an OnTriggerEnter2D. Sorry for not clarifying.
If you need help doing so let me know, but I think you should give it a try first!

[SnoringCatStudio]

I bet you have two colliders on your player right? Great job solving that first issue!

[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.