In short, the plugin does not look for "duplicate content". However, it allows to rollback a full import process. In other words, if something goes wrong or generates an unexpected result, almost all traces of the last import can be removed.
I think your question makes sense and probably, feature-wise, there could be some kind of checkbox that could add some extra steps before posting. However, that complicates things more than it seems. Some potential issues:
* Would two questions with the same title considered duplicate content?
* Would two questions with the same title and the same content but, the content differs in a single character be considered?
* Why would the plugin check for duplicate content while Q2A doesn't? (Note it only checks for duplicate content for specific posts, such as the latest 5 posts site-wise)
* Checking for duplicate titles and content will need additional indexes that Q2A does not support. Otherwise, the process would be very slow (and even with the indexes, the process would go slower because of these additional checks)
My conclusion is that unless there is a strong need of this feature (e.g. I receive that same request from many people) it is better to consider it as a no-go
