Try the following, Where DuplicateColumn would be the column that has John 5x and TableName would be your table
SELECT NomeColunaDuplicada, Count(*) as QtdRepeticoes FROM NomeTabela
GROUP BY NomeColunaDuplicada
HAVING Count(*) > 1
order by QtdRepeticoes desc;
This select will bring the record repeated and how many times it was repeated in descending order equal below:
Soon what has been repeated will be first, but if you want to return only use it:
SELECT NomeColunaDuplicada, Count(*) as QtdRepeticoes FROM NomeTabela
GROUP BY NomeColunaDuplicada
HAVING Count(*) > 1
order by QtdRepeticoes desc
limit 1;