The list of items in on the Items tab in the Panel Settings dialogue is inaccurate. Contains duplicate names along side different icons. Eg. "Clock" beside Whisker Menu icon. Also the name will change when hovering with the mouse pointer. Please see attachment image.
I'm using XFCE 4.12 and have the same issue with the panel item names being duplicated in the list, and they change when hovering the mouse over them. I am using Arch Linux.
I've found the cause of the issue, which explains why some people can't reproduce it but it happens for me.
I created a Docker image and everything seemed fine and couldn't reproduce this issue. Copied the same settings to my machine and I still had this issue. So I killed & ran xfce4-panel in the terminal. The issue is that I'm using the en_AU.utf8 locale.