Excel combine lambda and countif in formula

MichielS340 COUNTIF does not work with an array object in the range argument. You could try replacing COUNTIF(col;r) with SUM(N(col=r))>1. For example:

=LET(
    tbl; UNIQUE(VSTACK(HSTACK(tbl_Prognose[PRJ_NR];tbl_Prognose[DESC]);HSTACK(tbl_Orderportefeuille[Nummer];tbl_Orderportefeuille[Omschrijving])));
    col; CHOOSECOLS(tbl;1);
    FILTER(tbl;MAP(col;LAMBDA(r;SUM(N(col=r))>1));"")
)

An alternative approach without BYROW or MAP could be:

=LET(
    tbl; UNIQUE(VSTACK(HSTACK(tbl_Prognose[PRJ_NR];tbl_Prognose[DESC]);HSTACK(tbl_Orderportefeuille[Nummer];tbl_Orderportefeuille[Omschrijving])));
    col; CHOOSECOLS(tbl;1);
    only_once; UNIQUE(col;;1);
    FILTER(tbl;ISERROR(XMATCH(col;only_once));"")
)

See attached...