After you have read a tome with one username, if you change your username you can read it again.
How to Reproduce:
Read a tome with one username.
Change username with Mojang.
Read the same tome again.
Behold!
Please note, I am unable to reproduce this bug as the only reason I found it is because I changed my Mojang username. This bug was found on a multiplayer server.
Versions:
Description of a Bug:
After you have read a tome with one username, if you change your username you can read it again.
How to Reproduce: