We have multiple children and student events that require a large payment. A lot of our parents have more than one child. For example camp this year was $400. So we gave them an option to pay $100 at a time. Well after trying to figure out why we had so many duplicate children in our camp registration and constituents from this, I have reached out to case central and was informed that the registration is not set up to take multiple payments. (We have also had retreats for adults with multiple payments but because they were paying for themselves and not for another person the system didn't create a duplicate.) So it seems that children or payments for others are the only reason why the system is creating duplicates.
It would also be nice to have a way to select a different kind of payment with only one registration fee applied. (I may make this another idea to vote on, with more details on what I am talking about.)