XSLT selecting unique records

[IsValidUN]

How do you select unique records?
For example, if my data is like the following and I only want the repeated data (address information) to appear once for each theater_id.
<xsl:for-each select="Table"> ????



<Table>
<device_id>26 </device_id>
<serial_number> 200102</serial_number>
<is_active>true </is_active>
<commission_dat e>2006-01-10T00:00:00-05:00</commission_date >
<theater_id>000 00056</theater_id>
<theater_name>U ltra Star La Costa 6</theater_name>
<street_address 1>6941 El Camino Real</street_address1 >
<street_address 2 />
<street_city>Ca rlsbad</street_city>
<street_state>C A </street_state>
<street_zip_cod e>92009-4108</street_zip_code >
<street_country _id>USA</street_country_ id>
<screen_no>1</screen_no>
</Table>
<Table>
<device_id>27 </device_id>
<serial_number> 200713</serial_number>
<is_active>true </is_active>
<commission_dat e>2006-01-10T00:00:00-05:00</commission_date >
<theater_id>000 00056</theater_id>
<theater_name>U ltra Star La Costa 6</theater_name>
<street_address 1>6941 El Camino Real</street_address1 >
<street_address 2 />
<street_city>Ca rlsbad</street_city>
<street_state>C A </street_state>
<street_zip_cod e>92009-4108</street_zip_code >
<street_country _id>USA</street_country_ id>
<screen_no>1</screen_no>
</Table>

[jkmyoung]

http://www.jenitenniso n.com/xslt/grouping/muenchian.html

The easiest way would be something like:
<xsl:for-each select="Table[not (theater_id = previous-sibling::Table/theater_id)]">

A more efficient way would involve use of keys, see the link provided.

[IsValidUN]

Thanks!
I already tried your recommendation and it works.
I also looked at using the "keys" method but can't get it to work. I'll keep trying.

[jkmyoung]

Would look something like this:
<xsl:for-each select="Table[count(. | key('TableByThe aterid', theater_id)[1]) = 1]">