I’ve encountered an issue a few times in which my backpack inventory will show items in my toolbar, making it seem that I have more than one said item. Ended up selling my potato gun thinking I had two, but it ended up selling my only one 
This has been going on for a very long time and it happened to me too
It’s a recurring bug, that also prevents you from equipping items sometimes. Just drag an item into or out of your hotbar to get the game to update it.