sub test1 {
   my $len = length($_[0]);
   my $part1 = substr($_[0], 0, int($len/2));
   my $part2 = substr($_[0], -int($len/2));
   $_[0] = $part1 if ($part1 eq $part2);
}

sub test2 {
   $_[0] =~ s/^(.+)\s*\1/$1/;
}

sub test3 {
   $_[0] =~ s/(.{2,})\s*\1/$1/g;
}

sub test4 {
   $_[0] =~ s/\b(.+)\b\s*\1\b/$1/g;
}

foreach $test (qw( test1 test2 test3 test4 )) {
   print($test, "\n");

   foreach (
      'John SmithJohn Smith',
      'John Smith John Smith',
      'John Smith  John Smith',
      'foo John Smith John Smith bar',
      'John Johnson',
      'foo John Johnson bar',
      'John Smith!John Smith',
   ) {
      my $field = $_;
      &$test($field);
      print($field, "\n");
   }

   print("\n");
}

__END__
output
======
test1
John Smith
John Smith
John Smith  John Smith         <-- case not covererd
foo John Smith John Smith bar  <-- case not covererd
John Johnson
foo John Johnson bar           <-- case not covererd
John Smith                     <-- slightly buggy

test2
John Smith
John Smith
John Smith
foo John Smith John Smith bar  <-- case not covererd
Johnson                        <-- buggy
foo John Johnson bar           <-- case not covererd
John Smith!John Smith

test3
John Smith
John Smith
John Smith
foo John Smith bar
Johnson                        <-- buggy
foo Johnson bar                <-- buggy
John Smith!John Smith

test4
John Smith
John Smith
John Smith
foo John Smith bar
John Johnson
foo John Johnson bar
John Smith!John Smith