Place new Code to Working Code

Private Function isDuplicate(Target As Range) As Boolean
    isDuplicate = False
    If Target.Value <> "Applied For" Then
        If WorksheetFunction.CountIf(Me.Range("C11:C510"), Target.Value) > 1 Then
            Target.Select
            cEdit = MsgBox("You have enter the Enrolment No. <" & Target.Value & "> in <" & ShtName & ">" _
                           & vbNewLine & "is already exist. Click Ok to remove OR" _
                           & vbNewLine & "Click Cancel for Correction", vbExclamation + vbOKCancel + vbDefaultButton2, "Duplicate Entry!")
            
            isDuplicate = True
            Select Case cEdit
            Case Is = vbOK
                Target.ClearContents
            Case Is = vbCancel
                Target.Select
                Application.SendKeys "{F2}"
            End Select
        End If
    End If
    ' New Code for Duplication Entry ends here
End Function

Private Sub Worksheet_Change(ByVal Target As Range)

    If (Target.Count > 1) Then
        'too many cells are selected. your code is not supporting that
        Exit Sub
    End If
    
    If Intersect(Target, Me.Range("C11:C510")) Is Nothing Then Exit Sub
    
    On Error GoTo ReEnableEvents
    Application.EnableEvents = False

    Dim regExp As Object
    Dim strTarg As String
    Dim cEdit As Integer
    Dim i As Long
    Dim strPattern As String
    Dim arrTarg(1 To 4) As String
    Dim lngMidChrs As Long
    Dim ShtName As String
    Dim WrkbookName As String

    ShtName = ActiveSheet.Name
    WrkbookName = ActiveWorkbook.Name

    Set rngCheck = Range("D11:D510")
    Set rngBlock = Range("C11:C510")

    ' New Code for Empty Previous Cell of same Row (Seat No) starts here
    If Not Intersect(Target, Me.Range("C11:C510")) Is Nothing Then
        If Target.Value = "" Then GoTo ReEnableEvents
        If (IsEmpty(Target.Offset(0, -1))) Then
            Target.Select
            MsgBox "First enter Seat No. in <" & ShtName & ">", vbInformation + vbOKOnly, "Entry Required"
            Target.ClearContents
            GoTo ReEnableEvents
        End If
    End If
    
    ' New Code for Empty Previous Cell of same Row (Seat No)ends here

    'Test if user entered "Applied For" in lieu of Enrol No. code.
    'Test in upper case and if correct characters then convert to Proper case"
    If UCase(Trim(Target.Value)) = "APPLIED FOR" Then
        Target.Value = WorksheetFunction.Proper(Trim(Target.Value))
        GoTo ReEnableEvents    'Finished processing because "Applied for" entered
    
    ElseIf (isDuplicate(Target)) Then
        GoTo ReEnableEvents
    End If

    Set regExp = CreateObject("VBScript.RegExp")

    'Following line represents: '1st element plus Slash after 1st element
    '2nd element plus Slash after 2nd element
    '3rd element plus Slash after 3rd element
    '4th element

    'Convert alpha characters to upper case and remove spaces and slashes (if any)
    strTarg = UCase(Replace((Replace(Target.Value, " ", "")), "/", ""))
    strPattern = "^[A-Z]{6,7}[0-9]{8}$"    'Pattern to match

    If IsAMatch(regExp, strPattern, strTarg) Then
        'Insert the slashes in that pattern with 3 alphas in the second block
        If Len(strTarg) = 14 Then    'If 3 alpha + 3 alpha + 4 numeric + 4 numeric
            Target.Value = Left(strTarg, 3) & "/" & Mid(strTarg, 4, 3) & "/" & Chr(10) _
                           & Mid(strTarg, 7, 4) & "/" & Mid(strTarg, 11, 4)
        Else    'If 3 alpha + 4 alpha + 4 numeric + 4 numeric
            'insert the slashes in the pattern with 4 alphas in the second block
            Target.Value = Left(strTarg, 3) & "/" & Mid(strTarg, 4, 4) & "/" & Chr(10) _
                           & Mid(strTarg, 8, 4) & "/" & Mid(strTarg, 12, 4)
            GoTo ReEnableEvents
        End If

        'It will help to remove data if user enter enrolment no as student columns contains "Repeater(s)" Or "Improvement"
        If Intersect(Target, rngBlock) Is Nothing Then
            GoTo ReEnableEvents    ' Do nothing or what you want
        Else
            If Target.Offset(0, 1) = "Repeater(s)" Or Target.Offset(0, 1) = "Improvement" Then
                Target.Select
                MsgBox "As you enter <" & Target.Offset(0, 1) & "> in Student's Name Column in <" & ShtName & ">" _
                       & vbNewLine & "that is why you cannot enter Enrolment No.", vbInformation, "Information"
                Target.ClearContents
            End If
        End If
        Application.EnableEvents = True
        'ErrHandler:
    Else
        Target.Select
        cEdit = MsgBox("You entered <" & Target & "> is Invalid in <" & ShtName & ">" _
                       & vbNewLine & "Re-enter the Enrolment No. in one of the following description" _
                       & vbNewLine & "1. Write only -> applied for <- OR" _
                       & vbNewLine & "2. First write any 6 Alpha Characters and" _
                       & vbNewLine & " Second write any 8 Numeric Charaters OR" _
                       & vbNewLine & "3. First write any 7 Alpha Characters and" _
                       & vbNewLine & " Second write any 8 Numeric Charaters" _
                       & vbNewLine & " as provided by Enrolment Section." _
                       & vbNewLine & "4. Slashes may be omitted during entry." _
                       & vbNewLine & "Click Ok for Remove Enrolment no OR" _
                       & vbNewLine & "Click Cancel for Correction", vbCritical + vbOKCancel + vbDefaultButton2, "Invalid Entry!")
        If cEdit = vbOK Then
            Target.ClearContents
        End If
        If cEdit = vbCancel Then
            Target.Select
            Application.SendKeys "{F2}"
        End If

ReEnableEvents:
        If Err.Number <> 0 Then
            MsgBox "Error occurred in Private Sub Worksheet_Change." _
                   & vbNewLine & "Refer to Administrator Muneeb (KUBS - University of Karachi)" _
                   & vbNewLine & "of this workbook.", vbCritical, "Error!"
        End If
        Application.EnableEvents = True
    End If
    Application.EnableEvents = True
End Sub

Function IsAMatch(regEx As Object, strPatt As String, strToTest As String) As Boolean
'On Error GoTo ReEnableEvents
Dim regMatch As Object    ' MatchCollection

    With regEx
        .Pattern = strPatt
        .MultiLine = False
        .IgnoreCase = False    'Do NOT IgnoreCase. Set to True to Ignore Case
    End With

    'Match test string against regEx pattern string
    Set regMatch = regEx.Execute(strToTest)
    IsAMatch = (regMatch.Count > 0)

    Exit Function    'Do not re-enable events here unless error sends code to Sub routine

ReEnableEvents:
    If Err.Number <> 0 Then
        MsgBox "Error occurred in Function IsAMatch." _
               & vbCrLf & "Refer to Administrator of this workbook."
    End If
    Application.EnableEvents = True
End Function