Plain you here.
How do you post? From a mobile phone? From a PC or Mac or Linux computer? If so, which version of the operating system? Do you post using the standard or the HTML editor? Are those posts on NAT, or other sites. If NAT, how did you manage to have your post show the first "you" correctly, then?

You may have set up features such as AutoCorrect or AutoComplete, maybe something that was originally set to change "u" into "you" when "u" is a separate word but somehow changes the "u" when it's part of a full "you," thus resulting in "yoyou."