That's a new check to make sure the items that are being equipped are still the same items you it sent to be equipped. Do you have multiple windows open and are you changing equipment in one where you inventory has been modified in another?
Based on the numbers at the end, the id of the object it's trying to assign in your right hand doesn't isn't matching. That indicates something shifted in your inventory between when your inventory page was loaded and when you clicked Change Gear.
Like I said, this check isn't the ideal solution, but it involves a lot less rewriting of the the inventory page than the 'right' fix. And if I did it right, it shouldn't break inventory valid inventory changes. I'll be the first to admit that's a big if.
Yeah. You should NOT have been able to select the same one twice. Not sure how it happened, but because of the forced level ups. In theory if you had 2 new classes at the same time you might be able to reproduce it, but not going to devote much effort to it now. I'll add it to my list of things to fix though just in case, but probably won't be done til next age.