Deep research agents often rely on agreement among retrieved documents when deciding whether a claim is reliable. This heuristic breaks when the agreeing documents are not independent: several independent reports can support a claim, but several copies of the same false claim should not. We study evidence laundering, a content-only attack that publishes one false claim across paraphrased pages that appear to provide support from multiple sources. We evaluate this failure with deterministic matched corpora that present the same false claim and supporting propositions as a single source, a paraphrased correlated cluster, or an apparent independence control, with matched retrieval exposure. Experiments show that agents often assign laundered clusters credence comparable to the apparent independence control and become more willing to publish the false claim. Within the matched evaluation, a structural provenance gate substantially reduces this failure, suggesting that source independence modeling should precede answer synthesis.