how to find duplicated entries within a table?

[Gerhard Brauckmann]

Hello all,

I m trying to find duplicate entries in a specific table colum.
Does anyone of you have an idea how to do this by query?

example, my data: "John", "Willi", "Martha", "John", "Mike", "Sammy", "Mike"

Now i would select ONLY the double entries in this data-table.
The result should look like this:

John,John, Mike, Mike


any Idea is welcome! Thx & regards
Gerhard