Insert but ignore if duplicate AND set a value different if exist in another table
You can try in two steps as 

INSERT IGNORE INTO `invitations` (`iSender`, `iReceiver`) 
VALUES ('SENDER', 'RECEIVER');


This will ignore if the record already exists.

Then update with by joining the two tables

UPDATE `invitations` as i 
INNER JOIN users AS u on i.iReceiver = u.uOAuthID
SET `iProcessed` = 1 ;