Hello Nomnomno, and welcome to Seven Forums.
Ths pinned items on the taskbar will be located in this this
hidden system folder:
C:\Users\(User-Name)\AppData\Roaming\Microsoft\Internet Explorer\Quick Launch\User Pinned\TaskBar
You should be able to remove the duplicates from here. You may need to restart afterwards to apply though.
Hope this helps,
Shawn