What I expected to happen was...:
Two books of the same kind shouldn't be able to produce a new enchanted book if they don't increase the level when combined.
Fortune II + Fortune II => Fortune III (makes sense)
Fortune III + Fortune III => X (there is no fortune IV in survival)
What actually happened was...:
Two enchanted books of the same type makes one enchanted book of the same type, wasting a book.
Steps to Reproduce:
1. Place two enchanted books with the highest enchant available for that type in an anvil.
2. Take the output of those combined books (one enchanted book).
...yes. Intended.