unity – Method subscribed to an event is called twice for every time the event is raised

I am trying to make an inventory system in Unity with ScriptableObjects and a text driver that displays the contents of an inventory. So far the code looks like this:

-An InventoryObject is defined as having a “Container” List of ItemObjects (simple scriptable objects, they just have a little information on the object as its sprite, its name, and whether or not it is stackable) and some methods to properly add or remove items (taking item stacking into account) from that list. https://pastebin.com/t4P3aGmi

-A PlayerInventory is a script that will be carried by the player’s character. It contains a method for adding items collided with to its assigned InventoryObject instance. It also contains an OnItemPickedUp event that takes its InventoryObject instance as a parameter and passes it to listeners like UpdateDisplay. OnItemPickedUp is raised whenever a collision with a compatible item happens. https://pastebin.com/RsV7XYPp

-An UpdateDisplay script that listens to the OnItemPickedUp event and subscribes the
ProcessInventoryToSingleString(InventoryObject _inventory) method, that subsequently calls several methods to transform the contents of the InventoryObject’s Container list into a single, formatted string. https://pastebin.com/N0nsvcWH

-The items are represented by giving a GameObject with a collider2D, aSpriteRenderer and script that contains the particular ItemObject’s data and sets its SpriteRenderer’s sprite to the ItemObject’s.

The problem I’m having is that the method subscribed to the event is being called more than once for each time the event is raised. as shown by the debug logs. Sometimes it’s called twice, sometimes even four. This results in my UI display showing more items than there are.

Here are pictures of the console debug logs:
I'd copypaste the debug logs, but I can't do that in one go.

I appreciate any help.