=IF(COUNTIF(A:A,A1)1,"ERROR","")

HTH
--
AP

"Wilhelm" a écrit dans le message de
news: ...
I cannot figure out how to make an if function (or macro?) which returns
e.g.
the value false or error if the data in a certain cell already exists in
another cell in the same row.

I have made a table which shows which workers are working which day and at
which machine. The days are in the left column and the machines in the top
row. The workers(using last names) are in the cells inside the table. The
function should check or prevent that the same worker is working at two
different machines at the same time. In other words the same data cannot
be
in two different cells in the same row. Should I use some kind of if
function or would a macro work better?