  Rumor spreading on online social media is presenting a significant threat to
society of post-truth epoch. Extensive efforts have been devoted to rumor
identification and debunking, assuming that a specific rumor propagation is a
single event network and neglecting possible interdependence between different
rumor cascades. Here we study the collective propagation of multiple rumors,
and surprisingly find a network of users that repeatedly participate in
different rumor cascades. Though these repetitive users demonstrate minor
difference at the level of single propagation network, they are found to form a
significantly more intensive collaboration network from multiple rumor cascades
compared to news propagation. The clique-like cluster formed by repetitive
rumor spreaders can serve as a high quality feature for rumor identification
and blocking targets for rumor prevention. Our findings can provide a better
understanding of rumor spread by viewing multiple rumor propagations as one
interacting rumor ecosystem, and suggest novel methods for distinguishing and
mitigation based on rumor spreading history.
