Selected Answer
First off, "similar" names are not necessarily "same" names. To check for similarity, such as "Richard Smith" being recognised as "Richard Smithe", is possible, with a high degree of probability, using so-called regular expressions.
On the presumption that you actually mean to check if names are identical, there is no 100% sure way if both instances of each name was typed. Lots of hair has been pulled out over an accidental space added at the end. The thing to do is not to type a name twice at all. Instead, enter all the names in a list, load this list into data validation drop-downs or combo boxes, and let the user select the name from the drop-down. In this way the same name will always be the same name.
If this easy solution is not available to you the hard way would be to have code to do the checking. Please paste the code below in the code sheet of the worksheet on which you have the filtered list (look for Sheet1 (Sheet1) in VBE's Project Explorer and double-click on it to open the correct code sheet. It isn't necessaril.y Sheet1 and the name in parentheses is the name you gave to the tab.)
Option Explicit
Private Sub Worksheet_Calculate()
' 18 May 2018
Dim Rng As Range
Dim FirstName As Variant
Dim Cell As Range
Dim R As Long
R = Cells(Rows.Count, "D").End(xlUp).Row
Set Rng = Range(Cells(15, "D"), Cells(R, "D"))
For Each Cell In Rng.SpecialCells(xlCellTypeVisible)
If IsEmpty(FirstName) Then
FirstName = Cell.Value
Else
If Cell.Value <> FirstName Then
MsgBox "The filtered selection includes """ & FirstName & _
"""" & vbCr & " and at least one other name.", _
vbInformation, "Incorrect filter result"
Exit For
End If
End If
Next Cell
End Sub
This code will run when you apply the filter provided you have a SUM or SUBTOTAL function on the sheet which changes when the filter does. If you don't have such a function (sounds like you do) please add one. The trick is that the filter triggers the calculation of that total or subtotal and the act of calculation triggers the code. The code will stay quiet if all filtered items have the same name.