chsh corrupts /etc/pwd.db

[Alan Somers]

Today I tried to use chsh to change my shell from bash to fish.  The
command completed successfully, but new logins continued to use bash!
Investigating, I discovered that /etc/pwd.db and /etc/spwd.db seem to
contain 3-4 entries per user.  One of those still refers to my old
shell.  Worse, if I try using chsh again, it fails with an "entry
inconsistent" error, and I have to restore the password files from
backup.  Has anybody seen something like this before?  This is just a
single system, with no NIS or LDAP.

-Alan

$ xxd /etc/pwd.db
...
00003bf0: e800 0003 e800 0000 0000 416c 616e 2053  ..........Alan S
00003c00: 6f6d 6572 7300 2f68 6f6d 652f 736f 6d65  omers./home/some
00003c10: 7273 002f 7573 722f 6c6f 6361 6c2f 6269  rs./usr/local/bi
00003c20: 6e2f 6669 7368 0000 0000 0000 0003 df41  n/fish.........A
...
00005c80: 0041 6c61 6e20 536f 6d65 7273 002f 686f  .Alan Somers./ho
00005c90: 6d65 2f73 6f6d 6572 7300 2f75 7372 2f6c  me/somers./usr/l
00005ca0: 6f63 616c 2f62 696e 2f62 6173 6800 0000  ocal/bin/bash...
...
00006c20: 03e8 0000 03e8 0000 0000 0041 6c61 6e20  ...........Alan
00006c30: 536f 6d65 7273 002f 686f 6d65 2f73 6f6d  Somers./home/som
00006c40: 6572 7300 2f75 7372 2f6c 6f63 616c 2f62  ers./usr/local/b
00006c50: 696e 2f66 6973 6800 0000 0000 0000 03df  in/fish.........
...
00007b40: 0041 6c61 6e20 536f 6d65 7273 002f 686f  .Alan Somers./ho
00007b50: 6d65 2f73 6f6d 6572 7300 2f75 7372 2f6c  me/somers./usr/l
00007b60: 6f63 616c 2f62 696e 2f66 6973 6800 0000  ocal/bin/fish...

$ chsh -s /usr/local/bin/fish
Password:
chsh: entry inconsistent
chsh: pw_copy: Invalid argument