You can use the arrow keys to select items in the inventory when you have double-assigned letters like this. You could drop both items and pick them back up and hope that it does not keep a double assignment, or use the arrow keys to navigate to the item and try to access the item description menu that way to drop it or re-assign it. I do not know how re-assignment works if your inventory is already full, save first incase of a crash.
The default behavior of newer builds of the game as far as im aware is is to not assign something a letter after you hit 75 items. I read somewhere that clothing always gets assigned a letter however, so my guess as to what caused this is that you picked up/equipped something which maybe didnt have a letter, and then the game forced it to get a letter that had already been assigned? Or it was possibly double-assigned after some crafting happened. Needless to say having a full inventory has always been a pain. In older versions i encountered this bug on occasion, but was never certain of the cause.
If submitting a bug report for this, devs would need to know the version you are using because it may be fixed in recent builds. Its definitely a known problem for some older builds though.
