diff --git a/headroom/subscription/session_tracking.py b/headroom/subscription/session_tracking.py
index e9a1cacfd..9bcb557bb 100644
--- a/headroom/subscription/session_tracking.py
+++ b/headroom/subscription/session_tracking.py
@@ -134,6 +134,14 @@ def compute_window_tokens(start_ts: float, end_ts: float) -> WindowTokens:
     totals = WindowTokens()
     by_model: dict[str, WindowTokens] = {}
     unattributed = WindowTokens()
+    # Claude Code can store one assistant response across multiple transcript
+    # lines (e.g. one entry per content block), each carrying the SAME
+    # request-level ``message.usage``. Summing per line therefore multiplies a
+    # single response's tokens by its block count (observed 19x for one 420K
+    # response, #2340). Count each response's usage once, keyed by the unique
+    # Anthropic ``message.id``. Entries without an id keep the per-line
+    # behavior, so this only ever removes true duplicates.
+    seen_message_ids: set[str] = set()
 
     for path in find_transcript_files():
         # Skip transcripts that cannot contain entries inside the window.
@@ -171,6 +179,12 @@ def compute_window_tokens(start_ts: float, end_ts: float) -> WindowTokens:
             if not usage:
                 continue
 
+            msg_id = msg.get("id")
+            if isinstance(msg_id, str) and msg_id:
+                if msg_id in seen_message_ids:
+                    continue
+                seen_message_ids.add(msg_id)
+
             _add_usage_to_tokens(totals, usage)
 
             model_id: str | None = msg.get("model")
diff --git a/tests/test_subscription_session_tracking.py b/tests/test_subscription_session_tracking.py
index 058846ced..9074c7f58 100644
--- a/tests/test_subscription_session_tracking.py
+++ b/tests/test_subscription_session_tracking.py
@@ -87,3 +87,44 @@ def test_read_transcript_lines_preserves_small_transcript(
         '{"marker":"first"}',
         '{"marker":"second"}',
     ]
+
+
+def test_compute_window_tokens_dedups_usage_by_message_id(
+    monkeypatch: pytest.MonkeyPatch,
+    tmp_path: Path,
+) -> None:
+    """A single response stored across multiple transcript lines (one per content
+    block, same message.usage) must be counted once, not per line (#2340)."""
+    timestamp = "2026-01-01T00:00:00Z"
+    dup = {
+        "timestamp": timestamp,
+        "message": {
+            "id": "msg_dup",
+            "model": "claude-opus-4-1",
+            "usage": {"input_tokens": 100, "output_tokens": 10},
+        },
+    }
+    other = {
+        "timestamp": timestamp,
+        "message": {
+            "id": "msg_other",
+            "model": "claude-opus-4-1",
+            "usage": {"input_tokens": 5, "output_tokens": 2},
+        },
+    }
+    noid = {
+        "timestamp": timestamp,
+        "message": {"model": "claude-opus-4-1", "usage": {"input_tokens": 1, "output_tokens": 1}},
+    }
+    lines = [dup, dup, dup, other, noid]
+    transcript = tmp_path / "session.jsonl"
+    transcript.write_text("\n".join(json.dumps(e) for e in lines) + "\n", encoding="utf-8")
+    monkeypatch.setattr(session_tracking, "find_transcript_files", lambda: [transcript])
+
+    entry_ts = datetime.fromisoformat(timestamp.replace("Z", "+00:00")).timestamp()
+    tokens = session_tracking.compute_window_tokens(entry_ts - 1, entry_ts + 1)
+
+    # msg_dup counted once (100), msg_other (5), id-less line (1) -> 106, NOT 306.
+    assert tokens.input == 106
+    assert tokens.output == 13
+    assert tokens.by_model["claude-opus-4-1"]["input"] == 106