Duplicated rows when adding a measure to a table visual

The whole row is not duplicated.  What you see is powerbi giving you all the combinations of the dimension and fact table for which the measure returns a non-blank value.

This part

IF(
        TotalAntal = 0,
        0,

returns a 0 for all rows where the measure Beholdning minus udløbet is 0.  I don't know what that measure does but it looks like it returns 0 for all the combination that could be in the visual

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In powerbi, we sometimes use this feature to our advantage by adding 0 to a measure, for example, show me all the customers who have sales and also show me the customers who have no sales.  The measure above does something similar

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In any powerbi visual, when a measure returns blank, the rows will be removed from that visual.