diff -Naur mantis-1.1.1/bug_file_upload_inc.php mantis/bug_file_upload_inc.php
--- mantis-1.1.1/bug_file_upload_inc.php	2007-10-14 00:36:41.000000000 +0200
+++ mantis/bug_file_upload_inc.php	2008-01-20 23:46:25.000000000 +0100
@@ -57,8 +57,37 @@
 	<td width="85%">
 		<input type="hidden" name="bug_id" value="<?php echo $f_bug_id ?>" />
 		<input type="hidden" name="max_file_size" value="<?php echo $t_max_file_size ?>" />
-		<input name="file" type="file" size="40" />
-		<input type="submit" class="button" value="<?php echo lang_get( 'upload_file_button' ) ?>" />
+		<input name="file" type="file" size="40" id="file" />
+		<script type="text/javascript">
+			function check_filename() {
+				filename = document.getElementById('file');
+				filename = filename.value;
+				if(filename == '') {
+					return false;
+				}
+				lastslash = filename.lastIndexOf('/');
+				if(lastslash == -1) {
+					lastslash = filename.lastIndexOf('\\');
+				}
+				if(lastslash == -1) {
+					lastslash = 0;
+				}
+				else {
+					filename = filename.substring(lastslash + 1);
+				}
+				upload_allowed = true;
+				for(i in fileArray) {
+					if(fileArray[i] == filename) {
+						upload_allowed = false;
+					}
+				}
+				if(upload_allowed == false) {
+					alert(filename + ' already attached');
+					return false;
+				}
+			}
+		</script>
+		<input type="submit" class="button" onclick="return check_filename();" value="<?php echo lang_get( 'upload_file_button' ) ?>" />
 	</td>
 </tr>
 </table>
diff -Naur mantis-1.1.1/bug_view_page.php mantis/bug_view_page.php
--- mantis-1.1.1/bug_view_page.php	2007-10-14 00:36:41.000000000 +0200
+++ mantis/bug_view_page.php	2008-01-20 23:43:02.000000000 +0100
@@ -427,7 +427,7 @@
 		<?php echo lang_get( 'attached_files' ) ?>
 	</td>
 	<td colspan="5">
-		<?php file_list_attachments ( $f_bug_id ); ?>
+		<?php file_list_attachments ( $f_bug_id, true ); ?>
 	</td>
 </tr>
 <?php
diff -Naur mantis-1.1.1/core/file_api.php mantis/core/file_api.php
--- mantis-1.1.1/core/file_api.php	2007-12-20 06:46:29.000000000 +0100
+++ mantis/core/file_api.php	2008-01-20 23:43:58.000000000 +0100
@@ -145,7 +145,7 @@
 	# --------------------
 	# List the attachments belonging to the specified bug.  This is used from within
 	# bug_view_page.php and bug_view_advanced_page.php
-	function file_list_attachments( $p_bug_id ) {
+	function file_list_attachments( $p_bug_id, $p_with_js = false ) {
 		$t_attachment_rows = bug_get_attachments( $p_bug_id );
 
 		$num_files = sizeof( $t_attachment_rows );
@@ -159,11 +159,15 @@
 		$t_preview_image_ext = config_get( 'preview_image_extensions' );
 
 		$image_previewed = false;
+		$js = 'var fileArray = new Array();';
 		for ( $i = 0 ; $i < $num_files ; $i++ ) {
 			$row = $t_attachment_rows[$i];
 			extract( $row, EXTR_PREFIX_ALL, 'v' );
 
 			$t_file_display_name = string_display_line( file_get_display_name( $v_filename ) );
+			if ( $p_with_js ) {
+				$js .= 'fileArray[' . $i . '] = \'' . $t_file_display_name . '\';';
+			}
 			$t_filesize		= number_format( $v_filesize );
 			$t_date_added	= date( config_get( 'normal_date_format' ), db_unixtimestamp( $v_date_added ) );
 
@@ -281,6 +285,9 @@
 				PRINT "<br />\n";
 			}
 		}
+		if ( $p_with_js ) {
+			PRINT '<script type="text/javascript">' . $js . '</script>';
+		}
 	}
 	# --------------------
 	# delete all files that are associated with the given bug