Each time you receive an item from a visiting character (i.e. the Summer Character Visits), Steam “helpfully” generates a notification stating that the item is new to your inventory… even though the in-game notification (complete with sound effect!) shown at the bottom of the screen already does a good job of making this known to the player. In my experience, the Steam notification will usually be discovered after one is done playing, making it more of a nuisance than anything else.
Note that since this could be an issue on Valve’s end and not Pixeltail’s, there might not be a way to fix it by modifying TU’s code.
Also, I wasn’t sure whether to put this here or in Support/Steam Inventory.