Slots in hotbar are duplicating items. One item is cloned across hotbar. If you select new item in inventory it will be cloned to.
workaround for this problem is to drop all items and pick new from inventory, for each cloned slot because every time you do this process it's freeing one slot, sort off because this slot not working correctly afterwards, so its kinda severe, it's explained in this video.
Had this problem on two occasions in two different maps in creative mode only. First time it was three blocks cloned three times each, second time is in this vid it's one item cloned across eight slots.
device: Lenovo k50
[Mojang] Mega_Spud (Jay Wells) added gif: