MySQL: Selecting keys where column douplicate

That returns an error:
Error: Subquery returns more than 1 row

Well first of all, select you can always the first field of your original query:

SELECT bookkey from 
(SELECT bookkey, isbn, count(isbn) as cnt FROM elqBooks GROUP BY ISBN HAVING cnt > 1);

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A cleaned-up version:

SELECT bookkey FROM elqBooks GROUP BY ISBN HAVING count(*) > 1;

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but it should not work (nor your original query; did you try it?) since you aggregate without grouping by bookkey. So what is it *exactly* that you'd like to achieve?

-- for all ISBNs that appear more than once in the table, list all bookkeys?
SELECT bookkey from elqBooks 
where isbn in (select isbn from elqBooks group by isbn having count(*)>1);

-- list all bookkeys that appear more than once in the table?
SELECT bookkey FROM elqBooks GROUP BY bookkey HAVING count(*) > 1;

-- for all (bookkey, ISBN) pairs that appear more than once, list the bookkey only but not the isbn?
SELECT bookkey FROM elqBooks GROUP BY bookkey, ISBN HAVING count(*) > 1;

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