what insanity?

Duplicate Transfers (torrents with the same info hash) can't be added to the same instance of Tixati. Maybe you got 2 torrents with different hashes, but sharing identical (or similar) files. Those are not considered duplicate transfers.

it was with same info hash. i found duplicate transfer prompt disabled and had to enable it

Duplicate transfer prompt is only turned off by default for torrents automatically added from RSS. I guess it's because only users who are into cross seeding need to be notified in this case. Others might find it an unnecessary annoyance.