When the trace files shows duplicate Web.Content which might be triggered by some operations down the Power Query pipeline, it is a good idea to Buffer the Web.Contents first

Web.BufferedContents.m

(url) =>
  let 
    BufferedWithMetadata = (binary)=>(try Binary.Buffer() otherwise null) meta Value.Metadata(binary),
    response = BufferWithMetadata(Web.Contents(url,[ManualStatusHandling={404}])),
    out = if Value.Metadata(response)[Response.Status]=404 
          then null 
          else response
  in 
    out