newbie question, unique index || double entries

Hi,
I would like to create a unique index on two numerical fields in a table which has about 20'000 records.
I did a
SELECT COUNT(NO1||'.'||NO2)
FROM ...
WHERE ...
and a
SELECT COUNT(UNIQUE(NO1||'.'||NO2))
FROM ...
WHERE ...
Now the first SELECT reports 20'000
and the second reports 19'999
which means that I have _one_ DOUBLE ENTRY and my Index won't work! My question is, how do I spot the double entry? BIG thanks for your answer
Ma

    Marc A. Nussbaumer, Ph.D., Natural History Museum, Bernastrasse 15     CH-3005 Bern, Switzerland, Phone +41 (0)31 482290, FAX +41 31 482499

                       nussbaumer_at_nmbe.unibe.ch                                


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