Duplicate Names

I have a firstName text field on a form and a Surname Text field. on the Event AfterUpdate i have this code:

If (Not IsNull(DLookup("[FirstName]", _
"Employee", "[FirstName] ='" _
& Me!FirstName & "'"))) And (Not IsNull(DLookup("[Surname]", _
"Employee", "[Surname] ='" _
& Me!Surname & "'"))) Then
MsgBox "Someone already exists with the same name! Please check for duplicates", vbCritical, "IPDMS"
Me.Undo
End If

I am trying to Check the table "Employee" to prevent duplicate personnel from being entered.

My code at the moment is preventing any duplicate surname, its not combining it with the FirstName.

i.e. any person with the surname "Summers" cannot be entered twice even if they have different FirstName.

Thank you in advance.