Hi Ramie,
Were you perhaps trying to add a new value (the one that had already been added) for the same year? In this case the system will not allow multiple values for the same year. You should however be able to edit the value by clicking it (this will take you to a separate edit page) - let us know if you've tried this and encountered any troubles.
theresah.....2017-02-10 12:03:31 UTC