In the event below, If I enable the lines 'If Not doubleClickFlag Then ' and 'doubleClickFlag = False ' ,
then the following block of code does not run; 
' If IsNumeric(innerCell.Value) Then
                    If IsDate(innerCell.Offset(0, 2).Value) = False Then
                        MsgBox "Date Service Completed not present"
                        innerCell.Value = "" ' Reset the value to "" '

With the the following lines disabled 'If Not doubleClickFlag Then ' and 'doubleClickFlag = False ' ,
the following block code is looping continously;
' If IsNumeric(innerCell.Value) Then
                    If IsDate(innerCell.Offset(0, 2).Value) = False Then
                        MsgBox "Date Service Completed not present"
                        innerCell.Value = "" ' Reset the value to "" '

Here is the complete event that contains the problems;

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim rng As Range
    Dim cell As Range
    Dim innerCell As Range
    Set rng = Intersect(Target, Range("I:I"))
    If Not rng Is Nothing Then
        For Each cell In rng
            If IsDate(cell.Value) Then
                If cell.Offset(0, -7).Value = "Service" Then
                    MsgBox "Thank you for entering Service Date Completed. Please proceed to enter Service Schedule as the number of days."
                    Exit Sub
                Else
                    If Not doubleClickFlag Then
                        MsgBox "Please change the Issue description (column B) as required."
                        doubleClickFlag = False
                    End If
                    Exit Sub
                End If
            End If
        Next cell
    End If
    
    Set rng = Intersect(Target, Range("G:G"))
    If Not rng Is Nothing Then
        For Each innerCell In rng
            'If Not doubleClickFlag Then ' Check if message already displayed
                If IsNumeric(innerCell.Value) Then
                    If IsDate(innerCell.Offset(0, 2).Value) = False Then
                        MsgBox "Date Service Completed not present"
                        innerCell.Value = "" ' Reset the value to ""
                        Exit For ' Exit the loop
                        'doubleClickFlag = False ' Set flag to False
                    ElseIf innerCell.Offset(0, -5).Value <> "Service" Then
                        Exit For ' Exit the loop
                    Else
                        MsgBox "A New Service row will now be created"
                        innerCell.Offset(0, -5).Value = "Serviced"
                        innerCell.Offset(0, 1).Value = ""
                        
                        Dim newRow As Long
                        newRow = Cells(Rows.Count, "B").End(xlUp).Row + 1
                        Range("B" & newRow).Value = "Service"
                        Range("C" & newRow).Value = innerCell.Offset(0, 2).Value
                        Range("F" & newRow).Value = innerCell.Offset(0, -1).Value
                        Range("J" & newRow).Value = innerCell.Offset(0, 3).Value
                        Range("L" & newRow).Value = innerCell.Offset(0, 5).Value
                        Range("H" & newRow).Value = innerCell.Offset(0, 2).Value + innerCell.Value
                        Exit For ' Exit the loop
                    'End If
                End If
            End If
        Next innerCell
    End If
If Target.Column = 5 Then
ColumnList Target
End If
If Target.CountLarge > 1 Then Exit Sub
If Not Intersect(Target, Range("I:I")) Is Nothing Then
If IsDate(Target.Value) Then
If Range("B" & Target.Row).Value <> "Request" Then
Exit Sub
Else
MsgBox "Please change 'Request' to the relevant Task Issue"
Range("B" & Target.Row).Select
Exit Sub
End If
End If
End If
End Sub