diff --git a/strix/core/execution.py b/strix/core/execution.py
index 87a2bf02..fc37c18b 100644
--- a/strix/core/execution.py
+++ b/strix/core/execution.py
@@ -199,7 +199,14 @@ async def _salvage_stream_to_session(
     stream: Any,
     agent_id: str,
 ) -> None:
-    """Persist a crashed run's full history so a revived agent loses no context."""
+    """Persist a crashed run's history so a revived agent loses no context.
+
+    ``stream.to_input_list()`` reconstructs the conversation from this run cycle
+    as ``original_input + new_items``. After ``seed_initial_input``, the session
+    (pre_run_items) already contains the initial input on the first cycle, so
+    blindly concatenating duplicates it. We detect the overlap and append only
+    the delta.
+    """
     if stream is None:
         return
     try:
@@ -207,8 +214,12 @@ async def _salvage_stream_to_session(
     except Exception:
         logger.exception("could not build salvage history for %s", agent_id)
         return
-    desired = list(pre_run_items) + replay
-    if len(desired) <= len(pre_run_items):
+    pre_len = len(pre_run_items)
+    if pre_len > 0 and len(replay) >= pre_len and replay[:pre_len] == pre_run_items:
+        desired = replay
+    else:
+        desired = list(pre_run_items) + replay
+    if len(desired) <= pre_len:
         return
     try:
         await replace_session_items(session, desired)
diff --git a/tests/test_execution.py b/tests/test_execution.py
index 0daf0da2..a8d0f4ab 100644
--- a/tests/test_execution.py
+++ b/tests/test_execution.py
@@ -829,11 +829,22 @@ async def test_salvage_stream_to_session_preserves_full_history(tmp_path: Any) -
         "recon turn 2",
     ]
 
+    # A replayed full history starts with the already-persisted session items.
+    replayed_history = [
+        *stored,
+        {"role": "assistant", "content": "recovered turn"},
+    ]
+    await execution._salvage_stream_to_session(
+        session, stored, _SalvageStream(replayed_history), "child"
+    )
+    stored = [cast("dict[str, Any]", i) for i in await session.get_items()]
+    assert stored == replayed_history
+
     # A crash with nothing new to salvage leaves the session untouched.
     await execution._salvage_stream_to_session(
         session, list(await session.get_items()), _SalvageStream([]), "child"
     )
-    assert len(await session.get_items()) == 3
+    assert len(await session.get_items()) == len(replayed_history)
     session.close()