But: the point of this message is different. It is best for SEO to have focused threads that don’t repeat. It is also best from the point of view of finding knowledge: the less threads you need to find, the better.
When you start a new thread, Discourse gives you the closest thread titles available (however different they are), to give you the choice of continuing in one of these other threads, or starting a new thread altogether.
So, in other words, it is not a warning but a feature.