diff --git a/crates/aura-campaign/src/lib.rs b/crates/aura-campaign/src/lib.rs
index 4d72291..1c69c9f 100644
--- a/crates/aura-campaign/src/lib.rs
+++ b/crates/aura-campaign/src/lib.rs
@@ -237,6 +237,21 @@ pub fn preflight(process: &ProcessDoc, campaign: &CampaignDoc) -> Result<(), Exe
             StageBlock::Generalize { .. } => "std::generalize",
         }
     }
+    // Duplicate instruments enumerate identical cells whose registry family
+    // names collide (the name embeds the raw instrument string), racing one
+    // name's run-index assignment under the parallel cell loop — refused here
+    // as defense in depth beside the CLI's validate tier (#277 audit tidy).
+    let mut seen_instruments = std::collections::BTreeSet::new();
+    for instrument in &campaign.data.instruments {
+        if !seen_instruments.insert(instrument.as_str()) {
+            return Err(ExecFault::PipelineShape {
+                detail: format!(
+                    "campaign.data.instruments lists \"{instrument}\" more than once — \
+                     duplicate instruments collide on registry family names"
+                ),
+            });
+        }
+    }
     // shape: `std::sweep (std::gate)* (std::walk_forward)? (std::monte_carlo)?
     // (std::generalize)?` — a monotone rank walk over adjacent pairs captures
     // every violation: a rank drop is an out-of-order stage (an annotator
diff --git a/crates/aura-campaign/tests/execute.rs b/crates/aura-campaign/tests/execute.rs
index 9831606..19617eb 100644
--- a/crates/aura-campaign/tests/execute.rs
+++ b/crates/aura-campaign/tests/execute.rs
@@ -1177,3 +1177,25 @@ fn member_faults_stay_contained_in_the_parallel_loop() {
     assert!(out.record.cells.iter().all(|c| c.fault.is_some()));
     assert_eq!(reg.load_campaign_runs().expect("load runs").len(), 1);
 }
+
+/// Defense in depth (audit tidy, #277): `execute` itself refuses duplicate
+/// instruments via `preflight`, independent of the CLI's validate tier — a
+/// direct caller skipping `validate_campaign` must not reach the cell loop
+/// with colliding registry family names.
+#[test]
+fn execute_refuses_duplicate_instruments() {
+    let reg = temp_registry("duplicate_instruments");
+    let doc = campaign(&["AAA", "AAA"]);
+    let proc_doc = process(vec![sweep_stage(false)]);
+    let err = execute(
+        CAMPAIGN_ID,
+        &doc,
+        &proc_doc,
+        &strategies(),
+        &FakeRunner::clean(),
+        &reg,
+        DEFAULT_PARALLEL_INSTRUMENTS,
+    )
+    .expect_err("duplicate instruments collide on family names");
+    assert!(matches!(err, ExecFault::PipelineShape { .. }));
+}
diff --git a/crates/aura-cli/tests/research_docs.rs b/crates/aura-cli/tests/research_docs.rs
index 05d5284..75978e0 100644
--- a/crates/aura-cli/tests/research_docs.rs
+++ b/crates/aura-cli/tests/research_docs.rs
@@ -4075,3 +4075,12 @@ fn campaign_over_a_gapped_archive_records_the_uncovered_cell_and_continues() {
         "the campaign-run summary names the one failed cell: {out}"
     );
 }
+
+/// Acceptance pin (#277): `--parallel-instruments` rejects 0 at parse time —
+/// the NonZeroUsize carrier makes a zero residency bound unrepresentable, so
+/// clap refuses it as a usage error (exit 2) before any target resolution.
+#[test]
+fn campaign_run_rejects_a_zero_parallel_instruments_bound() {
+    let (out, code) = run_code(&["campaign", "run", "unused", "--parallel-instruments", "0"]);
+    assert_eq!(code, Some(2), "clap usage error expected, got: {out}");
+}
diff --git a/docs/design/INDEX.md b/docs/design/INDEX.md
index 501a28e..b543f81 100644
--- a/docs/design/INDEX.md
+++ b/docs/design/INDEX.md
@@ -125,6 +125,25 @@ same cell re-run under `aura generalize`) may differ by floating-point reassocia
 operation order (IEEE-754 non-associativity). This is not a C1 violation: C1 governs
 the determinism of a single run, not the cross-command bit-identity of a re-derived
 statistic.
+**Realization note (2026-07-16, #277).** Cross-sim parallelism now also spans
+campaign cells: the executor flattens the cell matrix, groups it by instrument
+ordinal, and walks sequential chunks of K instrument groups
+(`--parallel-instruments`, default 4 — a structural bound on distinct resident
+instruments, the RAM lever for the external data-server's per-reference file
+retention); within a chunk, cells run concurrently on the process-global rayon
+pool shared with the member/window fan-out. Results are collected into
+document-order slots, so outputs stay byte-identical across worker counts and
+bounds. Two deliberate scheduling-dependent carve-outs, both outside this
+contract's per-run bit-identity (which governs successful runs): on the
+run-fatal path (non-containable faults, e.g. a dead registry store) the
+propagated fault is the lowest document-order fault among the cells that
+completed before the abort flag latched, and the set of per-cell family lines
+already written by then is scheduling-dependent — inert, because no
+campaign-run record is written on that path and store reads are name-keyed,
+never line-ordered. Duplicate campaign instruments are refused at both the
+validate tier and the executor's preflight: the per-cell family name embeds
+the raw instrument string, so uniqueness is what keeps concurrent appends from
+racing one name's run-index assignment.
 **Forbids.** Concurrency *within* a single sim; any nondeterministic input that
 is not captured as an explicit input (see C11, C12).
 **Why.** Real money rides on backtest results; reproducibility and an audit