Hello Maxim,
I think may be you would need to fix that at the backend level or via Custom SQL to combine the date for each rows such as union.
yes there will huge number of rows and for any duplication you should be able to filter out at the dashboard level.
this way you may be able to count the events .
For some quick ideation see -How can I merge Creation and Deletion count from two different date series in a pivot? - #2 by robdhondt
Hope this helps.
Cheers,
Deep