Is there a way to ‘uniq’ by column?

sort -u -t, -k1,1 file

• -u for unique
• -t, so comma is the delimiter
• -k1,1 for the key field 1

Test result:

overflow@domain2.example,2009-11-27 00:58:29.793000000,xx3.net,255.255.255.0
stack2@domain.example,2009-11-27 01:05:47.893000000,xx2.net,127.0.0.1