Is there a way to construct a SQL2 query that would find a multi-value property containing duplicate values?
For instance, we have a multi-value String property called "language". Can a SQL2 query find a node which contains two or more instances of 'en' (English) in the "language" property?
There is no count(*) or group by selector in JCR SQL , XPath  or JCR-SQL2/AQM .
So you can't do it with SQL2 query but you can run query like below to find all the nodes with 'en' and then again query each node to check duplicate value using Node API
SELECT * FROM [nt:base] AS s WHERE ISDESCENDANTNODE([/content/AEM63App/fr]) and CONTAINS(s.language, 'en')