Hi,
I Think that the concept of an overlap is probably a bit hard, but it is really a matter of excel logic. What you probably could do is to create some kind of excel macro that copies the data from the XLS file to an XML-Excel-file which can then easily be imported to CRM.
Another option if you have more developer Resources is to create a VSTO addon in Excel that automatically from Excel exports the data from the excel sheets to CRM by using a button. It can validate the data Before this as well. It will require some development but would be very useful for the users as they would fill in the data and press the button, which would upload the data, nothing more need to be done.
I hope I have given you some ideas anyway.
