#!/usr/bin/perl -w
#<SortPW2.pl> /etc/passwd sort by ID

my $PASSWD="passwd";
my $fpid = open(PFILE, "<$PASSWD") or die "Password File Not Found: $!
+\n";

my %ids;
while (my $line = <PFILE>) {
   @ID = split(/:/, $line);
   my $id = ${ID[0]};
   if ($ids{$id}) {
       print "'$id' is duplicate\n";
   } else {
      $ids{$id} = 1;
   }
}
[download]

This is tangentially discussed in How can I remove duplicate elements from a list or array?, a perlfaq. You can construct your hash, and output whenever you hit a duplicate:

my %seen;
while (my $line = <PFILE>) {
   @ID = split(/:/, $line);
   my $id = $ID[0];
   if ($seen{lc($id)}++) {
       print "'$id' is duplicate\n";
   }
}
[download]

Note you had incorrect syntax for accessing a list element. I would also recommend against using variables with the same character sequence but different cases for names.

Just lowercase the $id you use for the hash lookup:

   my $id = $ID[0];
   my $id_lc = lc $id;
   if ($ids{$id_lc}) {
       print "'$id' is duplicate\n";
   } else {
      $ids{$id_lc} = 1;
   }
[download]