Remove duplicate error code and message

There were two separate error constants defined, ERROR_DUPLICATE_FILE
and ERROR_FILE_DUPLICATE, corresponding to an error message string with
identical text.

This commit removes the redundant constant ERROR_DUPLICATE_FILE and the
corresponding error message.

Fixes #5228

Conflicts:

	lang/strings_english.txt

core/constant_inc.php

@@ -214,7 +214,6 @@
 define( 'ERROR_NO_FILE_SPECIFIED', 4 );
 define( 'ERROR_FILE_DISALLOWED', 5 );
 define( 'ERROR_NO_DIRECTORY', 6 );
-define( 'ERROR_DUPLICATE_FILE', 9 );
 define( 'ERROR_DUPLICATE_PROJECT', 10 );
 define( 'ERROR_EMPTY_FIELD', 11 );
 define( 'ERROR_PROTECTED_ACCOUNT', 12 );

core/file_api.php

@@ -640,7 +640,7 @@ function file_add( $p_bug_id, $p_file, $p_table = 'bug', $p_title = '', $p_desc
 	}
 
 	if( !file_is_name_unique( $t_file_name, $p_bug_id ) ) {
-		trigger_error( ERROR_DUPLICATE_FILE, ERROR );
+		trigger_error( ERROR_FILE_DUPLICATE, ERROR );
 	}
 
 	if( 'bug' == $p_table ) {
@@ -1077,4 +1077,4 @@ function file_get_content_type_override( $p_filename ) {
 	$t_extension = pathinfo( $p_filename, PATHINFO_EXTENSION );
 
 	return $g_file_download_content_type_overrides[$t_extension];
-}
+}

lang/strings_english.txt

@@ -1508,7 +1508,6 @@ If you requested this verification, visit the following URL to change your passw
 	ERROR_NO_FILE_SPECIFIED => 'No file specified.',
 	ERROR_FILE_DISALLOWED => 'The file type is disallowed.',
 	ERROR_NO_DIRECTORY => 'The directory does not exist. Please check the project settings.',
-	ERROR_DUPLICATE_FILE => 'This is a duplicate file. Please delete the file first.',
 	ERROR_DUPLICATE_PROJECT => 'A project with that name already exists.',
 	ERROR_EMPTY_FIELD => 'A necessary field "%1$s" was empty. Please recheck your inputs.',
 	ERROR_PROTECTED_ACCOUNT => 'This account is protected. You are not allowed to access this until the account protection is lifted.',