Here below a data table of names:

Judge whether there are duplicate names, if so, fill in 1 in corresponding Dup field, otherwise fill in 0. Enter the formula in cell B2:

=spl("=if(?1.conj().select(~==?2).count()>1,1,0)",A$2:A$6,A2)

Then drag B2 down to every relevant row:

esProc Desktop and Excel Processing
8.9 Judging belongingness of sets when order is ignored
9.2 Judge duplication of row-based data - by key column