output in seconds.

This is how the data is looking like. Can you tell me the expected answer for any row and the logic followed to arrive at the answer?

 

Use this where Source needs to be replaced

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    ListPNR = List.Buffer(Source[PNR]),
    ListTime = List.Buffer(Source[Time]),
    CountTbl = Table.RowCount(Source),
    GenResultList = List.Generate(()=>[x=0,i=0], each [i]<CountTbl, each [i=[i]+1, x=if ListPNR{i+1}=ListPNR{i} then List.Max({Duration.From(0), ListTime{i}-ListTime{i+1}}) else 0 ], each try Duration.From([x]) otherwise Duration.From(0)),
    Result = Table.FromColumns(Table.ToColumns(Source) & {GenResultList}, Table.ColumnNames(Source) & {"Difference"})
in
    Result