Hello Mitsuhirato,

It sounds like something is resending your data twice.

Can you try adding &tmpl=component or ?tmpl=component to the URL, load the page and try again, does it work now?

Thanks
Tony

Alas it has no impact. I solved by modifying the edit.php.

Can you explain what you did - it may be a change we need to apply to the core code ??

Well, this solution fits to my purpose but maybe is not suitable for everyone.
When saving I fetch the last to entries in the database (jevents_vevdetail and jevents_vevent are needed), compare the title and the date. If title is the same and date differs no more than 2 seconds, the last one those two entries will be deleted (in both mentioned tables).

Did you see the overlapping event restrictions option - you can use this to block creating events that occur at the same time and this can be setup to be true universally or within specific categories.

To be honest: no. But now I tested the function with the result, that the event cannot be saved at all (one still remains on the edit page).