With a portal, you have some kind of relationship linking the parent record to the related portal records such as:
ParentTable::PrimaryKey = PortalTable::ForeignKey
Let's call the date field in the portal "Date".
Define a text field to combine date and ForeignKey in an auto-entered calculations:
ForeignKey & " " & Date
Set a unique values validation rule on this field and your user will get an error message each time they enter a date that is already listed in the portal.