List of duplicates in a List

Hi,
I have a list of object with some having a duplicate field value. Say, I have List<myObj> where myObj might look something like field1, field2, date, myID with values:
blah, blahblah, 2/2/2009, 4
yada, yadayada, 2/4/2009, 4
where myID might contain duplicate values. I need to be able to get a list that contains the first one (with the oldest date) so I can remove them from the list and be left with "yada" record, or just be able to remove the oldest one without getting the list of duplicates.
Thanks.
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