> The trigger approach works like a charm. Thanks for the ideas...

But trigger approach is not safe in multiuser environment.

You may want to add another column with unique constraint,
and fill that column with CRC checksum in trigger.
(if you do not mind using UDF)

Ivan
http://www.volny.cz/iprenosil/interbase/

Another good idea is to trim replies - more than 80 lines
for single line answer ...