Create a unique index on the employee + date.

1. Open your table in design view.

2. Open the Indexes dialog (View menu.)

3. Enter on the first blank row in the dialog, enter a name for the index
(anything you like) in the first column, and in the 2nd column select the
employee field name.

4. In the lower pane of the dialog set the Unique property to Yes.

5. On the next row of the dialog, leave the index name column blank, and in
the 2nd column select the WeekEndingDate field. The 2 rows of the dialog
will look something like this:
   EmployeeIdWeekEndingDate    EmployeeID
                                                WeekEndingDate

6. Save the changes.

This will prevent you from entering 2 records for the same person and date.

You probably don't have a field named "Name". That will cause interminable
problems if you do, because almost every object in Access has a Name
property. You probably have a table of employees with an EmployeeID (like
Northwind does).