Please describe the problem:
It's possible to add the same tag multiple times, with tracker-tag.

Steps to reproduce:
moogman@gbcdff1461m:~$ touch file
moogman@gbcdff1461m:~$ tracker-tag file
/home/moogman/file:
moogman@gbcdff1461m:~$ tracker-tag file -a test
moogman@gbcdff1461m:~$ tracker-tag file -a test
moogman@gbcdff1461m:~$ tracker-tag file -a test
moogman@gbcdff1461m:~$ tracker-tag file -l
/home/moogman/file: test test test

Actual results:
The tag "test" gets assigned to the file multiple times.

Expected results:
That the tag "test" gets assigned to the file once. Any further attempts should not add the same tag.

Does this happen every time?
Yes

Other information:
moogman@gbcdff1461m:~$ trackerd --version
Tracker version 0.5.4 Copyright (c) 2005-2006 by Jamie McCracken (jamiemcc@gnome.org)

moogman@gbcdff1461m:~$ apt-cache show tracker | grep Version
Version: 0.5.4-4

moogman@gbcdff1461m:~$ cat /etc/lsb-release 
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=7.04
DISTRIB_CODENAME=feisty
DISTRIB_DESCRIPTION="Ubuntu feisty (development branch)"