Snapshot 12w50a, Minecraft 1.4.6, Snapshot 13w05b, Snapshot 13w07a, Minecraft 1.5
Windows, Java 7 u10
#NOTE - THIS ONLY EFFECTS SURVIVAL#
The anvil will not allow you to combine two enchanted books of the same enchantment in survival.
In the previous snapshot, it was possible to combine enchanted books this way and get a higher level enchantment. (i.e. two sharpness IV books created 1 Sharpness V).
See picture for more details.
This issue relates to MC-4578. Whether it is intended to be able to combine enchanted books or not, the issue described in
MC-4578 should not be happening in survival mode.