Try the following, Where Columnupled Name would be the column that has John 5x and Table Name 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 repeated record and how many times it was repeated in equal descending order below:
Soon the one that was most repeated will be in first, but in case you want to return only it use:
SELECT NomeColunaDuplicada, Count(*) as QtdRepeticoes FROM NomeTabela
GROUP BY NomeColunaDuplicada
HAVING Count(*) > 1
order by QtdRepeticoes desc
limit 1;