Incremental Refresh Error: Duplicates in primary key (even after remove duplicates)

Hi folks,

I´m having the following problem with incremental refresh: 

My model includes a table in which I create a unique key by concatinating a date column with an id column (which results in multiple rows in the key column) and then adding a step "remove duplicates" which (as expected) results in a unique key column.

However, as soon as I use incremental refresh (keep last 10 days and refresh / add 1 day) I get an error message saying that there are duplicates in this column. 

Is it possible that the "remove duplicates" step is not performed on the whole data at the end (containing historical and refreshed data) but that it is actually performed once for historical data and once for new data, which are then appended resulting in remaining duplicates?

How would you suggest to prevent that from happening?

Please find below the m-code of the table in question with the relevant part highlighted in red.

Thanks a lot in advance,

Nico

M Code:

let

DatenAb=Text.From(Date.From(RangeStart)),
DatenBis=Text.From(Date.From(RangeEnd)),
Mandant="AbsatzPrognose",
Segment=Segmentfilter,

Quelle = Oracle.Database(Datenbank,
[Query=
"SELECT
mandant,
commodity,
vertragsnummer,
segment,
vertragsnehmer,
vertragsnehmer_knr,
anschlussnehmer,
anschlussnehmer_knr,
zp_id,
malo_id,
malo_zaehlpunktbezeichnung,
malo_marktlokation_ident,
rgzs_name,
land,
ort,
plz,
strasse,
haus_nr,
haus_nr_e,
zaehlverfahren,
ht_arbeit,
nt_arbeit,
kundenwert,
prog_param_entn_kurzfrist,
prog_param_entn_mittelfrist,
prog_param_entn_langfrist_norm,
prog_param_entn_langfrist_kalt,
prog_param_entn_langfrist_warm,
prog_param_eins_kurzfrist,
prog_param_eins_mittelfrist,
prog_param_eins_langfrist_norm,
prog_param_eins_langfrist_kalt,
prog_param_eins_langfrist_warm,
tag

FROM VIM_OWNER.V_INSIGHT_PROGNOSE_CHECK pp
WHERE pp.MANDANT = '"& Mandant &"'
AND pp.SEGMENT = '"& Segment &"'

AND pp.TAG >= to_date('"&DatenAb&"', 'dd.mm.yyyy')
AND pp.TAG < to_date('"&DatenBis&"', 'dd.mm.yyyy')
"
, HierarchicalNavigation=true]),
#"Gefilterte Zeilen" = Table.SelectRows(Quelle, each [TAG] >= RangeStart and [TAG] < RangeEnd),
#"Monatsbeginn eingefügt" = Table.AddColumn(#"Gefilterte Zeilen", "Monatsbeginn", each Date.StartOfMonth([TAG]), type datetime),
#"Geänderter Typ" = Table.TransformColumnTypes(#"Monatsbeginn eingefügt",{{"Monatsbeginn", type date}}),
#"Zusammengeführte Spalte eingefügt" = Table.AddColumn(#"Geänderter Typ", "Key_Malo_Monat", each Text.Combine({Text.From([MALO_ID]), Text.From([Monatsbeginn], "de-DE")}, ""), type text),
#"Entfernte Duplikate" = Table.Distinct(#"Zusammengeführte Spalte eingefügt", {"Key_Malo_Monat"})
in
#"Entfernte Duplikate"