Ubuntu – Invalid system activity (sysstat) file on Ubuntu server

I'm periodically getting this message from the daily cron on my server. (I don't get this message every day.)

/etc/cron.daily/sysstat:
Invalid system activity file: /var/log/sysstat//sa28

Here's some basic info on my server:

Ubuntu 12.04.2 LTS
Linux server 3.8.4-x86_64-linode31 #1 SMP Mon Mar 25 16:00:34 EDT 2013 x86_64 x86_64 x86_64 GNU/Linux

Here is the relevant directory listing:

/var/log/sysstat$ ls -lai
total 44
6601 drwxr-xr-x  2 root root 4096 Apr 29 06:48 .
6550 drwxr-xr-x 13 root root 4096 Apr 29 06:48 ..
  16 -rw-r--r--  1 root root  384 Apr 22 00:00 sa21
 510 -rw-r--r--  1 root root  384 Apr 23 00:00 sa22
 524 -rw-r--r--  1 root root  384 Apr 24 00:00 sa23
2613 -rw-r--r--  1 root root  384 Apr 25 00:00 sa24
2199 -rw-r--r--  1 root root  384 Apr 26 00:00 sa25
2745 -rw-r--r--  1 root root  384 Apr 27 00:00 sa26
2577 -rw-r--r--  1 root root  384 Apr 28 00:00 sa27
  22 -rw-r--r--  1 root root  384 Apr 29 00:00 sa28
1221 -rw-r--r--  1 root root  336 Apr 29 21:45 sa29
2765 -rw-r--r--  1 root root    0 Apr 22 06:55 sar21
2744 -rw-r--r--  1 root root    0 Apr 23 06:39 sar22
2003 -rw-r--r--  1 root root    0 Apr 24 06:27 sar23
1974 -rw-r--r--  1 root root    0 Apr 25 06:36 sar24
1600 -rw-r--r--  1 root root    0 Apr 26 06:25 sar25
2753 -rw-r--r--  1 root root    0 Apr 27 06:54 sar26
 423 -rw-r--r--  1 root root    0 Apr 28 06:54 sar27
 526 -rw-r--r--  1 root root    0 Apr 29 06:48 sar28

I have no idea what the problem is and I don't even understand why I'm seeing two copies of each file name. (I see they are different inodes but it is still confusing.)

My first step was to read this: http://linux.die.net/man/1/sar

I appreciate any suggestions.