Just looking through my mantis configuration and this maybe important: $g_html_make_links is set to ON (which is the default).
Maybe this is causing the double HTML link generation i am seeing?

Hi nicr/DGtlRift,

I believe i've now fixed this issue in CVS:

--- custom_field_api.php 16 Sep 2004 16:48:53 -0000 1.42
+++ custom_field_api.php 17 Nov 2004 19:57:55 -0000
@@ -942,7 +942,7 @@
$result = str_replace( '||', '', '|' . $p_value . '|' );
break;
default:

• $result = string_display_links( $p_value );
• $result = $p_value;
  }
  return $result;
  }
  @@ -1150,7 +1150,7 @@
  return str_replace( '|', ', ', $t_custom_field_value );
  break;
  default:
• return $t_custom_field_value;
• return string_display_links( $t_custom_field_value );
  }
  }

Feel free to apply the diff above, and see if this resolves the issue for you.

Paul