What I gave you previously works for a separate app (like Word or Excel).
This works for killing the second instance of the same app and activating/maximizing the first instance:
Code:
Option Explicit

Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare Function ShowWindow Lib "user32" (ByVal hwnd As Long, ByVal nCmdShow As Long) As Long

Private Const SW_SHOWMAXIMIZED = 3

Private Sub Form_Load()
Dim lhwnd As Long
Dim lRtrn As Long
Dim sFrmCaptn As String
Dim X As Form

    'Get Caption to Search For
    sFrmCaptn = Me.Caption
    Me.Caption = ""

    'Search for Window / Get its Handle
    lhwnd = FindWindow(vbNullString, sFrmCaptn)
    
    'If Another Window Found
    If lhwnd <> 0 Then
        'Activate and Maximize Window
        lRtrn = ShowWindow(lhwnd, SW_SHOWMAXIMIZED)
        
        'Kill App
        For Each X In Forms
            Unload X
            Set X = Nothing
        Next
        Unload Me

    'If No Other Window Found    
    Else
        'Set Caption back to Original Caption
        Me.Caption = sFrmCaptn
    End If
End Sub