diff --git a/home-mixer/filters/retweet_deduplication_filter.rs b/home-mixer/filters/retweet_deduplication_filter.rs
index 1216f5a..71c015a 100644
--- a/home-mixer/filters/retweet_deduplication_filter.rs
+++ b/home-mixer/filters/retweet_deduplication_filter.rs
@@ -31,8 +31,11 @@ impl Filter<ScoredPostsQuery, PostCandidate> for RetweetDeduplicationFilter {
                 }
                 None => {
                     // Mark this original tweet ID as seen so retweets of it get filtered
-                    seen_tweet_ids.insert(candidate.tweet_id as u64);
-                    kept.push(candidate);
+                    if seen_tweet_ids.insert(candidate.tweet_id as u64) {
+                        kept.push(candidate);
+                    } else {
+                        removed.push(candidate);
+                    }
                 }
             }
         }
@@ -40,3 +43,70 @@ impl Filter<ScoredPostsQuery, PostCandidate> for RetweetDeduplicationFilter {
         Ok(FilterResult { kept, removed })
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::candidate_pipeline::candidate::PostCandidate;
+    use crate::candidate_pipeline::query::ScoredPostsQuery;
+
+    #[tokio::test]
+    async fn test_retweet_deduplication_filter_removes_original_if_retweet_seen_first() {
+        let filter = RetweetDeduplicationFilter;
+        let query = ScoredPostsQuery::default();
+
+        let retweet_candidate = PostCandidate {
+            tweet_id: 100,
+            retweeted_tweet_id: Some(200),
+            ..Default::default()
+        };
+
+        let original_candidate = PostCandidate {
+            tweet_id: 200,
+            retweeted_tweet_id: None,
+            ..Default::default()
+        };
+
+        let candidates = vec![retweet_candidate, original_candidate];
+
+        let result = filter.filter(&query, candidates).await.unwrap();
+
+        // Should keep the retweet (first occurrence of 200)
+        assert_eq!(result.kept.len(), 1);
+        assert_eq!(result.kept[0].tweet_id, 100);
+
+        // Should remove the original tweet (second occurrence of 200)
+        assert_eq!(result.removed.len(), 1);
+        assert_eq!(result.removed[0].tweet_id, 200);
+    }
+
+    #[tokio::test]
+    async fn test_retweet_deduplication_filter_removes_retweet_if_original_seen_first() {
+        let filter = RetweetDeduplicationFilter;
+        let query = ScoredPostsQuery::default();
+
+        let original_candidate = PostCandidate {
+            tweet_id: 200,
+            retweeted_tweet_id: None,
+            ..Default::default()
+        };
+
+        let retweet_candidate = PostCandidate {
+            tweet_id: 100,
+            retweeted_tweet_id: Some(200),
+            ..Default::default()
+        };
+
+        let candidates = vec![original_candidate, retweet_candidate];
+
+        let result = filter.filter(&query, candidates).await.unwrap();
+
+        // Should keep the original tweet (first occurrence of 200)
+        assert_eq!(result.kept.len(), 1);
+        assert_eq!(result.kept[0].tweet_id, 200);
+
+        // Should remove the retweet (second occurrence of 200)
+        assert_eq!(result.removed.len(), 1);
+        assert_eq!(result.removed[0].tweet_id, 100);
+    }
+}