Dynamic Index

The code below will start the Index column at the first Non-Null in the FM+ column.

Read the code comments as the code can be simplified if nulls will ONLY be at the beginning of the column and not interspersed within the column.

Since it does not depend on a Join (merge), there is no issue with duplicates.

let
   Source = Table.FromColumns(
       {{1..9},
        {null,null, 2,5,9,14,null,27,35}},
        type table[FM=Int64.Type, #"FM+"=Int64.Type]),

//Add index starting at first non-blank entry in FM+

//use next line if there will never be nulls except at the beginning of the column
//    #"FM+ 1st" = Table.RowCount(Source) - List.NonNullCount(Source[#"FM+"]),

//if nulls could be anywhere but we need to start at the First non-null then
    #"FM+ 1st" = 
        List.Max(
            List.Accumulate(
                List.PositionOf(Source[#"FM+"],null,Occurrence.All),
                {}, (state, current)=>
                if state = {} and current=0 then {0}
                    else if List.Last(state) + 1 = current then state & {current}
                    else state & {null}
        ))+1,

    #"Index List" = List.Repeat({null}, #"FM+ 1st") & {1..Table.RowCount(Source) - #"FM+ 1st"},
    #"Add Index" =  Table.FromColumns(
        Table.ToColumns(Source) & {#"Index List"},
        type table[FM=Int64.Type, #"FM+"=Int64.Type, Index=Int64.Type]
    )
in 
    #"Add Index"

Example