hotfix: assertion to guarantee that new features are not known

src/processor/processor_tracker_feature.cpp

@@ -47,7 +47,10 @@ unsigned int ProcessorTrackerFeature::processNew(const int& _max_new_features)
 
     // fill the track matrix
     for (auto ftr : new_features_last_)
+    {
+        assert(std::find(known_features_last_.begin(), known_features_last_.end(), ftr) == known_features_last_.end() && "detectNewFeatures() provided a new feature that is already in known_features_last_`");
         track_matrix_.newTrack(ftr);
+    }
 
     // Track new features from last to incoming. This will append new correspondences to matches_last_incoming
     trackFeatures(new_features_last_,