Only once, because key and email address are verified when you set your name for the credits. However …
I bought 78 $ Collector’s Co-op Expansion Bundle and I set two name for two keys with same e-mail adress . This is a problem ?
Depends on what you consider a problem … if I’m not completely mistaken, then the second name overwrote the first entry.
Edit: Turns out I was mistaken.
But if this is true very big problem because they are two different keys and one name for me and second name for my friend . If second name overwrote my friend’s name overwrote to my name . How I can fix this ?
No answer 
Probibly by registering the other key to your friends e-mail by contacting them and getting it changed over. I believe you can re-enter your name with your key it just overwrites whatever entry you last put in.
I tried with different e-mail for my friend but not it require original e-mail . If set two name for two different key with same e-mail owerwrote another one what I can make ?
EDIT: De-Ninja’d - please see Rhis’s post below 
EDIT:VVVVV See Rhis’ post below.VVVVV
You don’t need to change the email addresses, each key gets it’s own name. If you have more than one key then they don’t overwrite each other.
Finally my stress was end
I am too scared 