On my bards I manually set the mainhand weapon, so I only need to swap offhand items. Everything works fine untill the offhand item is identical to mainhand item. In this case, what appears to be happening is, when the offhand item is swaped, it finds the mainhand item and swaps it to the offhand. The item with the same name from inventory bags is left in the bags, leaving the mainhand empty. I also tested this swapping out both hands with the same result, bard with only offhand equiped when items are identical. I tested on my warrior, same results. The only resolution appears to be making sure you don't dual wield identical weapons.
Note: Items with same name crash /findslot as well. See : Staff in primary and secondary hand. ( I took one out and it stopped crashing.)
Sorry for delay Chance, is was Simple Defiant Bastard Sword on bard. forget what I used on warrior, was a couple cheap items I bought just to test.