I tried your suggestion, but it did not work properly if I had to use more that two columns (Invoice Date 1; Invoice Date 2 ... Invoice Date 4)
It allows loading the values even with 4 Inovice dates correctly (checked in a table box).
But if I set up a bar chart in which I use Month([Invoice Date]) as a dimension, and sum([Invoice Value]), in each month that has more than two values the sums are not calculated properly (see month June, July, Oct.)
Only months in which I do have 1 or 2 entries are shown correct values. (Please see screen shot).
The suggestion by Ani to use "inner join" worked in this setting very well.