diff --git a/crates/engine/src/events.rs b/crates/engine/src/events.rs
index fbc1b9b6c..05433412c 100644
--- a/crates/engine/src/events.rs
+++ b/crates/engine/src/events.rs
@@ -127,12 +127,15 @@ pub fn topics(v: Option<&Value>) -> Result<Vec<String>, String> {
     for t in list {
         let Some(s) = t.as_str() else { return Err("`topics` must be an array of strings".into()) };
         match s {
+            // Each once: the list is scanned for every retained event under the log's lock, so
+            // a long list of repeats would multiply that work.
+            "comments" | "document" | "all" if out.iter().any(|t| t == s) => {}
             "comments" | "document" | "all" => out.push(s.to_string()),
             other => return Err(format!("unknown topic `{other}` (comments, document, all)")),
         }
     }
-    if out.is_empty() {
-        out.push("all".into());
+    if out.is_empty() || out.iter().any(|t| t == "all") {
+        out = vec!["all".into()];
     }
     Ok(out)
 }
diff --git a/crates/engine/src/events_tests.rs b/crates/engine/src/events_tests.rs
index 36b118a79..9cdef52ca 100644
--- a/crates/engine/src/events_tests.rs
+++ b/crates/engine/src/events_tests.rs
@@ -193,6 +193,10 @@ fn topics_parse_and_reject_unknown_names() {
     assert_eq!(events::topics(Some(&json!([]))).unwrap(), vec!["all".to_string()]);
     assert_eq!(events::topics(Some(&json!(["comments", "document"]))).unwrap(), vec!["comments".to_string(), "document".to_string()]);
     assert!(events::topics(Some(&json!(["nope"]))).is_err());
+    // Repeats collapse (the list is scanned per event under the log's lock), and `all` is all.
+    let many: Vec<&str> = std::iter::repeat_n("comments", 10_000).chain(["document"]).collect();
+    assert_eq!(events::topics(Some(&json!(many))).unwrap(), vec!["comments".to_string(), "document".to_string()]);
+    assert_eq!(events::topics(Some(&json!(["comments", "all", "document"]))).unwrap(), vec!["all".to_string()]);
     assert!(events::topics(Some(&json!("comments"))).is_err());
 }