it's not possible to post a topic if topics with duplicate names are not allowed and a topic with the same name is deleted.
is it a bug or a feature?

Not to dredge up an oldish topic, I seem to be having a similar issue.
I Noticed after deleting a topic and selecting remove from database that I could not create a topic with the same name again. After enabling "Allow Topics With Same Name" to Registered Users and All Users I was still unable to create the topic again or create a new set of topics with the same names on version 1.9.5.5 (Date: 3/3/2011)