diff --git a/services/aggregation/worker.py b/services/aggregation/worker.py
index 39cab62..102c657 100644
--- a/services/aggregation/worker.py
+++ b/services/aggregation/worker.py
@@ -841,7 +841,11 @@ async def persist_trend_evidence(
     supporting: list[RankedEvidence],
     opposing: list[RankedEvidence],
 ) -> int:
-    """Insert evidence mapping rows for a trend window. Returns count inserted."""
+    """Insert evidence mapping rows for a trend window. Returns count inserted.
+
+    Deletes any existing evidence for this trend window first to prevent
+    duplicate accumulation across aggregation cycles.
+    """
     rows: list[tuple[str, str, str, float, float, float, float, float, float]] = []
     for ev in supporting:
         # Skip non-UUID document IDs (e.g. pattern signal synthetic IDs)
@@ -861,6 +865,12 @@ async def persist_trend_evidence(
             ev.recency_component, ev.confidence_component, ev.sentiment_value,
         ))
 
+    # Clear stale evidence before inserting fresh rows
+    await pool.execute(
+        "DELETE FROM trend_evidence WHERE trend_window_id = $1",
+        trend_window_id,
+    )
+
     if not rows:
         return 0