Maybe something like the following,

Sub AttDownload()
Dim oItem As Object
Dim oAtt As Object
Dim sName As String
Dim sExt As String
Dim sPath As String
    sPath = Environ("USERPROFILE") & "\Desktop\SUmmary\"
    For Each oItem In CreateObject("Outlook.Application").GetNamespace("MAPI").getdefaultfolder(6).Items
        If TypeName(oItem) = "MailItem" Then
            If Format(oItem.ReceivedTime, "mmmm yyyy") = Format(Date, "mmmm yyyy") Then
                For Each oAtt In oItem.attachments
                    If LCase(oAtt.FileName) Like "*calibration*" Then
                        sName = oAtt.FileName
                        sExt = Right(sName, Len(sName) - InStrRev(sName, Chr(46)))
                        sName = FileNameUnique(sPath, sName, sExt)
                        oAtt.SaveAsFile sPath & sName
                    End If
                Next oAtt
            End If
        End If
    Next oItem
    Set oAtt = Nothing
    Set oItem = Nothing
End Sub

Private Function FileNameUnique(strPath As String, _
                               strFileName As String, _
                               strExtension As String) As String
'Graham Mayor - http://www.gmayor.com - Last updated - 22 Jun 2018
'strPath is the path in which the file is to be saved
'strFilename is the filename to check
'strextension is the extension of the filename to check
Dim lng_F As Long
Dim lng_Name As Long
Dim FSO As Object
    Set FSO = CreateObject("Scripting.FileSystemObject")
    Do Until Right(strPath, 1) = "\"
        strPath = strPath & "\"
    Loop
    If InStr(1, strFileName, "\") > 0 Then
        strFileName = Mid(strFileName, InStrRev(strFileName, "\") + 1)
    End If
    strExtension = Replace(strExtension, Chr(46), "")
    lng_F = 1
    lng_Name = Len(strFileName) - (Len(strExtension) + 1)
    strFileName = Left(strFileName, lng_Name)
    'If the filename exists, add or increment a number to the filename
    'and keep checking until a unique name is found
    Do While FSO.FileExists(strPath & strFileName & Chr(46) & strExtension) = True
        strFileName = Left(strFileName, lng_Name) & "(" & lng_F & ")"
        lng_F = lng_F + 1
    Loop
    'Reassemble the filename
    FileNameUnique = strFileName & Chr(46) & strExtension
lbl_Exit:
    Set FSO = Nothing
    Exit Function
End Function