Parents occasionally make mistakes and will select the wrong child, so it's not really a duplicate. 
You will be able to see this as the records will have different names:
To address this, please click on the "Edit" icon and "Reset Member".
Then send them a new invite and add an explanation in the text box of the invite to assist families in knowing what they did incorrectly.
Be sure to also send the a reconfirm to the other sibling record as well - as you won't be able to tell which information is for which sibling.