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:

https://c.scudata.com/article/1651916536524
https://c.scudata.com/article/1655564899392
https://c.scudata.com/article/1655595734583