If the drawing already contains the block, you need to call InsertBlock
with the block name only. Something like this:
Dim exist as Boolean
exist = False
For i = 0 To ThisDrawing.Blocks.Count - 1
If StrComp(ThisDrawing.Blocks.Item(i).Name, "blocks", vbTextCompare) = 0 Then
exist = True
Exit For
End If
Next i
ThisDrawing.ModelSpace.InsertBlock(point, IIf(exist, "blocks", "D:\blocks.dwg"), 1, 1, 1, 0)