Concatenate cell values for duplicate numbers

This should do it

Sub test1()
    Dim sTemp As String, r As Long, lastrow As Long
    lastrow = Range("A" & Rows.Count).End(xlUp).Row
    With CreateObject("Scripting.Dictionary")
        For r = 2 To lastrow
            sTemp = Range("A" & r)
            If Not .Exists(sTemp) Then
                .Item(sTemp) = Range("B" & r)
            Else
                .Item(sTemp) = .Item(sTemp) & ", " & Range("B" & r)
            End If
        Next r
        For r = 2 To lastrow
            sTemp = Range("A" & r)
            Range("B" & r) = .Item(sTemp)
        Next r
    End With
    Range("A1:B" & lastrow).RemoveDuplicates Columns:=Array(1, 2), Header:=xlYes
End Sub

Sorry, didn't spot that you were on Mac

This should work on Mac (untested)

Sub test2()
    Dim sTemp As String, r As Long, lastrow As Long, sArray() As String, nMaxID As Double
    lastrow = Range("A" & Rows.Count).End(xlUp).Row
    nMaxID = WorksheetFunction.Max(Range("A:A"))
    ReDim sArray(nMaxID)
        For r = 2 To lastrow
            If sArray(Range("A" & r)) = "" Then
                sArray(Range("A" & r)) = Range("B" & r)
            Else
                sArray(Range("A" & r)) = sArray(Range("A" & r)) & ", " & Range("B" & r)
            End If
        Next r
        For r = 2 To lastrow
            Range("B" & r) = sArray(Range("A" & r))
        Next r
    Range("A1:B" & lastrow).RemoveDuplicates Columns:=Array(1, 2), Header:=xlYes
End Sub

Here is a formula solution using several helper columns:
Enter in C2 and fill down and across to column E

Formula:

=IF(COUNTIF($A$2:$A2,$A2)=COUNTIF($A$2:$A$19,$A2),"",B3)

Enter in F2 and fill down

Formula:

=IF(C2="","",A2)

Enter in G2 and fill down to enter a , between the values concatenated.

Formula:

=SUBSTITUTE(TRIM(IF(C2="","",B2&" "&C2&" "&D2&" "&E2))," ",", ")

Enter in I2 and fill across and down to get the unique P/N values with the associated H-D#s

Formula:

=IFERROR(INDEX(F$2:F$19,MATCH(1,INDEX((COUNTIF($I$1:I1,$F$2:$F$19)=0)*($F$2:$F$19<>""),0),0)),"")