diff --git a/CHANGELOG.md b/CHANGELOG.md
index c8ec9465..c8272042 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Avoid duplicate sidebar rows when a compressed session completes after both the preserved snapshot id and continuation id are already present in the session list.
+
 ## [v0.51.171] — 2026-05-30 — Release EQ (stage-batch53 — tool-output card badge + Neon opt-in skin)
 
 ### Added
diff --git a/static/sessions.js b/static/sessions.js
index 0c89c1da..bc5af517 100644
--- a/static/sessions.js
+++ b/static/sessions.js
@@ -361,7 +361,9 @@ function _markSessionCompletedInList(session, previousSid = null) {
   if (!session || !Array.isArray(_allSessions)) return;
   const finalSid = session.session_id || previousSid;
   if (!finalSid) return;
-  const idx = _allSessions.findIndex(s => s && (s.session_id === finalSid || s.session_id === previousSid));
+  const finalIdx = _allSessions.findIndex(s => s && s.session_id === finalSid);
+  const previousIdx = previousSid ? _allSessions.findIndex(s => s && s.session_id === previousSid) : -1;
+  const idx = finalIdx >= 0 ? finalIdx : previousIdx;
   if (idx < 0) return;
   const {messages: _messages, tool_calls: _toolCalls, ...sessionMeta} = session;
   const messageCount = Number(
@@ -384,6 +386,11 @@ function _markSessionCompletedInList(session, previousSid = null) {
   _sessionStreamingById.set(finalSid, false);
   _forgetObservedStreamingSession(finalSid);
   if (previousSid && previousSid !== finalSid) {
+    for (let i = _allSessions.length - 1; i >= 0; i--) {
+      if (i !== idx && _allSessions[i] && _allSessions[i].session_id === previousSid) {
+        _allSessions.splice(i, 1);
+      }
+    }
     _sessionStreamingById.delete(previousSid);
     _forgetObservedStreamingSession(previousSid);
     _sessionListSnapshotById.delete(previousSid);
diff --git a/tests/test_issue856_background_completion_unread.py b/tests/test_issue856_background_completion_unread.py
index 2a62e935..2f885a88 100644
--- a/tests/test_issue856_background_completion_unread.py
+++ b/tests/test_issue856_background_completion_unread.py
@@ -100,7 +100,9 @@ def test_sidebar_cache_completion_handles_compression_session_rotation():
 
     assert "function _markSessionCompletedInList(session, previousSid = null)" in helper_block
     assert "const finalSid = session.session_id || previousSid;" in helper_block
-    assert "s.session_id === finalSid || s.session_id === previousSid" in helper_block
+    assert "const finalIdx = _allSessions.findIndex(s => s && s.session_id === finalSid);" in helper_block
+    assert "const previousIdx = previousSid ? _allSessions.findIndex(s => s && s.session_id === previousSid) : -1;" in helper_block
+    assert "const idx = finalIdx >= 0 ? finalIdx : previousIdx;" in helper_block
     assert "const {messages: _messages, tool_calls: _toolCalls, ...sessionMeta} = session;" in helper_block
     assert "...sessionMeta" in helper_block
     assert "session_id: finalSid" in helper_block
diff --git a/tests/test_sidebar_completion_dedupe.py b/tests/test_sidebar_completion_dedupe.py
new file mode 100644
index 00000000..82d26c44
--- /dev/null
+++ b/tests/test_sidebar_completion_dedupe.py
@@ -0,0 +1,51 @@
+import json
+import subprocess
+from pathlib import Path
+
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+
+
+def _extract_js_function(src: str, name: str) -> str:
+    start = src.index(f"function {name}(")
+    brace = src.index("{", start)
+    depth = 0
+    for idx in range(brace, len(src)):
+        ch = src[idx]
+        if ch == "{":
+            depth += 1
+        elif ch == "}":
+            depth -= 1
+            if depth == 0:
+                return src[start : idx + 1]
+    raise AssertionError(f"could not extract {name}")
+
+
+def test_mark_session_completed_in_list_dedupes_old_and_new_sid_rows():
+    sessions_src = (REPO_ROOT / "static" / "sessions.js").read_text(encoding="utf-8")
+    fn_src = _extract_js_function(sessions_src, "_markSessionCompletedInList")
+    script = f"""
+let _allSessions = [
+  {{ session_id: 'old', title: 'Before', message_count: 10, pre_compression_snapshot: true }},
+  {{ session_id: 'new', title: 'After', message_count: 2, parent_session_id: 'old' }},
+];
+let _sessionStreamingById = new Map([['old', true], ['new', true]]);
+let _sessionListSnapshotById = new Map([['old', {{message_count: 10}}]]);
+function _forgetObservedStreamingSession(_sid) {{}}
+function renderSessionListFromCache() {{}}
+{fn_src}
+_markSessionCompletedInList({{
+  session_id: 'new',
+  title: 'After done',
+  message_count: 3,
+  parent_session_id: 'old',
+}}, 'old');
+console.log(JSON.stringify({{rows: _allSessions, oldStreaming: _sessionStreamingById.has('old')}}));
+"""
+    result = subprocess.run(["node", "-e", script], check=True, text=True, capture_output=True)
+    payload = json.loads(result.stdout)
+
+    assert [row["session_id"] for row in payload["rows"]] == ["new"]
+    assert payload["rows"][0]["title"] == "After done"
+    assert payload["rows"][0]["message_count"] == 3
+    assert payload["oldStreaming"] is False