Merged queries is duplicating values

You have 'Cardboard' and 'Plastic' in your Month and Units table, so you get a month for each of those Materials, therefore you're getting 19 months * 3 rows in Lines table * 2 Materials in Month and Units table = 114 rows total.

I don't really understand why you need to duplicate the rows in the Lines table for each month? Can you use a DimDate table and some measures to achieve this result instead?

Otherwise try referencing the Month & Units table in a new query, do a Remove Duplicates on the Month column and use that for your merge.

Advanced Editor M script for the referenced query:

let
Source = Table2,
#"Removed Duplicates" = Table.Distinct(Source, {"Month"})
in
#"Removed Duplicates"

Where Table2 is your Month & units table.

Then use that new Query (name it DistinctMonths ? or something that makes sense to you) as your second table in the merge.