Hi Matthias,

Kindly find my response for your inquiries below:

When inviting friends we run into a problem, when we try to invite a friend with the same dress for a second time ... no E-Mail is created

This is actually the default behavior, the email address that already entered previously will be skip. Where you actually can resend the invitation email by head to friend>sent invitation: http://take.ms/bytSO

Also, why is there n option to invite Friends via Social Platforms (we would need Facebook urgently!)

Unfortunately, it is no option to invite friends via social platforms currently. Perhaps, can you create a new ticket under feature request so our developers can keep track the feature that can be implement in the next release