why does it happened?

i backup my databases with innobackupex-1.5.1, and this is error log:

InnoDB: Error: trying to add tablespace 126 of name ‘./fb/relation_0.ibd’
InnoDB: to the tablespace memory cache, but tablespace
InnoDB: 126 of name ‘./fb/login_19.ibd’ already exists in the tablespace
InnoDB: memory cache!
innobackupex-1.5.1: Error: ibbackup child process has died at /usr/bin/innobackupex-1.5.1 line 336.

why does it happened?