diff --git a/docs/content/docs/features/parallelism.mdx b/docs/content/docs/features/parallelism.mdx
index 836a066ff..0a4d493d4 100644
--- a/docs/content/docs/features/parallelism.mdx
+++ b/docs/content/docs/features/parallelism.mdx
@@ -131,13 +131,21 @@ sim = MajoranaPropagator(..., comm=MPI.COMM_SELF)
 
 ### Multi-node (`MPI.COMM_WORLD`)
 
-Replace the communicator and launch with `mpiexec`:
+Pass a duplicate of `MPI.COMM_WORLD` and launch with `mpiexec`:
 
 ```python notest
 from mpi4py import MPI
-sim = MajoranaPropagator(..., comm=MPI.COMM_WORLD)
+comm = MPI.COMM_WORLD.Dup()
+sim = MajoranaPropagator(..., comm=comm)
+...
+comm.Free()  # once sim and its functionals are no longer used
 ```
 
+monoprop exchanges point-to-point messages on fixed tags over `comm`, so your own sends and receives
+on the same communicator could match them; a receive with `MPI.ANY_TAG` could even take one of
+monoprop's messages and leave it waiting. Reserve the duplicate for monoprop, with no point-to-point
+traffic of your own on it. It must outlive the propagator and every functional obtained from it.
+
 ```bash
 mpiexec -n 8 uv run python your_script.py
 ```
diff --git a/src/monoprop/majorana_propagator.py b/src/monoprop/majorana_propagator.py
index 4ad371e53..0072ea386 100644
--- a/src/monoprop/majorana_propagator.py
+++ b/src/monoprop/majorana_propagator.py
@@ -111,7 +111,10 @@ def __init__(
                 exception applies on top of either.
             lower_atol: Monomials with ``|coeff| < lower_atol`` are discarded during evolution.
             upper_atol: Monomials with ``|coeff| > upper_atol`` are kept regardless of complexity.
-            comm: Optional MPI communicator (must outlive the simulator).
+            comm: Optional MPI communicator (must outlive the simulator). Pass a duplicate
+                reserved for monoprop, such as ``MPI.COMM_WORLD.Dup()``: monoprop exchanges
+                point-to-point messages on fixed tags, which your own sends and receives on the
+                same communicator, including ``MPI.ANY_TAG`` receives, could match.
         """
         majorana_operator: MajoranaOperator = (
             initial_operator
diff --git a/src/monoprop/monomial_propagator.py b/src/monoprop/monomial_propagator.py
index ca3a711cc..5b840e492 100644
--- a/src/monoprop/monomial_propagator.py
+++ b/src/monoprop/monomial_propagator.py
@@ -104,7 +104,10 @@ def _init_simulator(
             lower_atol: Coefficient-truncation threshold, or ``None``.
             upper_atol: Coefficient-retention threshold, or ``None``.
             basis_change: Internal per-Majorana basis change (``2 * num_modes`` entries).
-            comm: Optional MPI communicator (must outlive the propagator).
+            comm: Optional MPI communicator (must outlive the propagator). Pass a duplicate
+                reserved for monoprop, such as ``MPI.COMM_WORLD.Dup()``: monoprop exchanges
+                point-to-point messages on fixed tags, which your own sends and receives on the
+                same communicator, including ``MPI.ANY_TAG`` receives, could match.
             basis: Engine basis -- ``"majorana"`` (default) or ``"pauli"``.
         """
         num_modes = majorana_operator.num_modes
diff --git a/src/monoprop/pauli_propagator.py b/src/monoprop/pauli_propagator.py
index ec8719262..009bf4b33 100644
--- a/src/monoprop/pauli_propagator.py
+++ b/src/monoprop/pauli_propagator.py
@@ -70,7 +70,10 @@ def __init__(
                 so a value approaching ``num_qubits`` is rejected with a ``RuntimeError`` rather than started.
             lower_atol: Monomials with ``|coeff| < lower_atol`` are discarded during evolution.
             upper_atol: Monomials with ``|coeff| > upper_atol`` are kept regardless of weight.
-            comm: Optional MPI communicator (must outlive the propagator).
+            comm: Optional MPI communicator (must outlive the propagator). Pass a duplicate
+                reserved for monoprop, such as ``MPI.COMM_WORLD.Dup()``: monoprop exchanges
+                point-to-point messages on fixed tags, which your own sends and receives on the
+                same communicator, including ``MPI.ANY_TAG`` receives, could match.
         """
         # The engine takes the Schrodinger cutoff in slots (two per qubit); this API in qubits.
         schrodinger_cutoff = (
diff --git a/tests/conftest.py b/tests/conftest.py
index c6750a97e..e5c700a37 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -20,6 +20,12 @@
 allreduce internally. ``serial_comm`` is always COMM_SELF: use it for rank-local state such as
 ``evolved_operator``, ``contract_partially``, ``size()``, or ``graph_size()``.
 
+Both hand out session-wide duplicates of those communicators, never the predefined handles
+themselves, as the parallelism docs tell users to do: monoprop's point-to-point traffic then
+cannot match messages that conftest or mpi4py exchange on the predefined communicators.
+Each duplicate is created on first use and freed at session teardown: ``Dup`` is collective, and
+doing it per test would add a collective to every one of them for no extra isolation.
+
 Under more than one rank each rank gets its own ``--basetemp``; see ``_rank_basetemp``.
 """
 
@@ -27,16 +33,21 @@
 
 import getpass
 import os
+import stat
 import tempfile
 from pathlib import Path
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING
 
 import pytest
 
-from tests.mpi_support import HAS_MPI_BACKEND, MPI, SERIAL_COMM
+from tests.mpi_support import HAS_MPI_BACKEND, MPI
 from tests.reference_data import load_problem
 
 if TYPE_CHECKING:
+    from collections.abc import Iterator
+
+    from mpi4py.MPI import Intracomm
+
     from tests.reference_data import FermionicProblem
 
 
@@ -58,6 +69,28 @@ def _rank_basetemp() -> Path | None:
     return temproot / f"pytest-of-{user}" / f"mpi-rank-{MPI.COMM_WORLD.rank}"
 
 
+def _make_private_dir(path: Path) -> None:
+    """Create ``path`` if needed and make sure only the current user can use it.
+
+    The checks pytest applies to its own ``pytest-of-<user>``, which an explicit ``--basetemp``
+    skips: on a shared temporary root another user could create the predictable name first.
+    """
+    path.mkdir(mode=0o700, parents=True, exist_ok=True)
+    # Not following symlinks: a swapped-in link would otherwise pass every check below.
+    info = path.lstat()
+    if stat.S_ISLNK(info.st_mode):
+        raise OSError(
+            f"The temporary directory {path} is a symbolic link. Fix this and try again."
+        )
+    if hasattr(os, "getuid") and info.st_uid != os.getuid():
+        raise OSError(
+            f"The temporary directory {path} is not owned by the current user. "
+            "Fix this and try again."
+        )
+    if info.st_mode & 0o077:
+        path.chmod(0o700)
+
+
 @pytest.hookimpl(tryfirst=True)
 def pytest_cmdline_main(config: pytest.Config) -> None:
     """Give each MPI rank its own ``--basetemp`` unless one was passed explicitly.
@@ -67,7 +100,7 @@ def pytest_cmdline_main(config: pytest.Config) -> None:
     """
     if config.option.basetemp is None and (basetemp := _rank_basetemp()) is not None:
         # pytest creates only the basetemp itself when one is given, not its parent.
-        basetemp.parent.mkdir(mode=0o700, parents=True, exist_ok=True)
+        _make_private_dir(basetemp.parent)
         config.option.basetemp = basetemp
 
 
@@ -124,26 +157,53 @@ def pytest_runtest_setup(item: pytest.Item) -> None:
             pytest.skip(f"Test requires {min_size} MPI processes")
 
 
+@pytest.fixture(scope="session")
+def comm_self() -> Iterator[Intracomm | None]:
+    """Session-wide duplicate of COMM_SELF; ``None`` when monoprop has no MPI backend."""
+    if not HAS_MPI_BACKEND:
+        yield None
+        return
+    duplicate = MPI.COMM_SELF.Dup()
+    yield duplicate
+    duplicate.Free()
+
+
+@pytest.fixture(scope="session")
+def comm_world() -> Iterator[Intracomm | None]:
+    """Session-wide duplicate of COMM_WORLD; ``None`` when monoprop has no MPI backend."""
+    if not HAS_MPI_BACKEND:
+        yield None
+        return
+    duplicate = MPI.COMM_WORLD.Dup()
+    yield duplicate
+    duplicate.Free()
+
+
 _COMM_PARAMS = (
     [pytest.param(None, id="comm_none")]
     if not HAS_MPI_BACKEND
     else [
-        pytest.param(MPI.COMM_SELF, id="comm_self"),
-        pytest.param(MPI.COMM_WORLD, id="comm_world"),
+        pytest.param("comm_self", id="comm_self"),
+        pytest.param("comm_world", id="comm_world"),
     ]
 )
 
 
 @pytest.fixture(params=_COMM_PARAMS)
-def comm(request: pytest.FixtureRequest) -> Any:  # noqa: ANN401
-    """Parametrized communicator; ``None`` when mpi4py is unavailable or monoprop has no MPI backend."""
-    return request.param
+def comm(request: pytest.FixtureRequest) -> Intracomm | None:
+    """Parametrized communicator; ``None`` when mpi4py is unavailable or monoprop has no MPI backend.
+
+    Resolved through the session-scoped duplicates, so parametrizing does not reorder the run.
+    """
+    if request.param is None:
+        return None
+    return request.getfixturevalue(request.param)
 
 
 @pytest.fixture
-def serial_comm() -> Any:  # noqa: ANN401
+def serial_comm(comm_self: Intracomm | None) -> Intracomm | None:
     """Single-rank communicator for tests that inspect rank-local operator state."""
-    return SERIAL_COMM
+    return comm_self
 
 
 @pytest.fixture(
diff --git a/tests/mpi_support.py b/tests/mpi_support.py
index be9e3217a..79a9d3222 100644
--- a/tests/mpi_support.py
+++ b/tests/mpi_support.py
@@ -16,8 +16,6 @@
 
 from __future__ import annotations
 
-from typing import Any
-
 import monoprop
 
 try:
@@ -27,6 +25,3 @@
 
 HAS_MPI_BACKEND: bool = MPI is not None and bool(monoprop.MPI_ABIS)
 """True when mpi4py is importable and this monoprop installation ships an MPI backend."""
-
-SERIAL_COMM: Any = MPI.COMM_SELF if HAS_MPI_BACKEND else None
-"""Single-rank communicator for rank-local checks; ``None`` selects the serial backend."""
diff --git a/tests/test_evolved_operator_coefficients.py b/tests/test_evolved_operator_coefficients.py
index a2419a9cb..a28bf7a05 100644
--- a/tests/test_evolved_operator_coefficients.py
+++ b/tests/test_evolved_operator_coefficients.py
@@ -33,7 +33,6 @@
 )
 from monoprop.majorana import Majorana, MajoranaOperator
 from monoprop.pauli import Pauli, PauliOperator
-from tests.mpi_support import SERIAL_COMM
 from tests.reference_data import load_problem
 
 DATA = Path(__file__).parent / "data"
@@ -48,7 +47,7 @@ def problem():
 
 
 @pytest.fixture(scope="module")
-def prop_and_params(problem):
+def prop_and_params(problem, comm_self):
     """A built propagator over ``problem`` and its parameters, shared across the module.
 
     ``evolved_operator`` and ``evolved_operator_coefficients`` leave simulator state alone, so one
@@ -58,7 +57,7 @@ def prop_and_params(problem):
         problem.operator,
         problem.monomial_circuit.initial_state,
         cutoff=2 * problem.n_modes,
-        comm=SERIAL_COMM,
+        comm=comm_self,
     )
     prop.build_graph(problem.monomial_circuit.to_circuit())
     return prop, problem.monomial_circuit.parameters
diff --git a/tests/test_mpi_configuration.py b/tests/test_mpi_configuration.py
index 8668b5066..34a6cd886 100644
--- a/tests/test_mpi_configuration.py
+++ b/tests/test_mpi_configuration.py
@@ -159,3 +159,38 @@ def test_basetemp_parent_is_created(monkeypatch, tmp_path):
     config.option.basetemp = None
     conftest.pytest_cmdline_main(config)
     assert Path(config.option.basetemp).parent.is_dir()
+
+
+def _basetemp_for_rank(monkeypatch, temproot: Path) -> Path:
+    monkeypatch.setattr(conftest, "MPI", Mock(COMM_WORLD=Mock(size=3, rank=2)))
+    monkeypatch.setenv("PYTEST_DEBUG_TEMPROOT", str(temproot))
+    config = Mock()
+    config.option.basetemp = None
+    conftest.pytest_cmdline_main(config)
+    return Path(config.option.basetemp)
+
+
+def test_basetemp_parent_owned_by_another_user_is_refused(monkeypatch, tmp_path):
+    monkeypatch.setattr(conftest.os, "getuid", lambda: tmp_path.stat().st_uid + 1)
+    with pytest.raises(OSError, match="not owned by the current user"):
+        _basetemp_for_rank(monkeypatch, tmp_path)
+
+
+def test_basetemp_parent_symlink_is_refused(monkeypatch, tmp_path):
+    temproot = tmp_path / "temproot"
+    temproot.mkdir()
+    elsewhere = tmp_path / "elsewhere"
+    elsewhere.mkdir(mode=0o700)
+    monkeypatch.setattr(conftest.getpass, "getuser", lambda: "someone")
+    (temproot / "pytest-of-someone").symlink_to(elsewhere)
+    with pytest.raises(OSError, match="symbolic link"):
+        _basetemp_for_rank(monkeypatch, temproot)
+
+
+def test_basetemp_parent_is_made_private(monkeypatch, tmp_path):
+    monkeypatch.setattr(conftest.getpass, "getuser", lambda: "someone")
+    parent = tmp_path / "pytest-of-someone"
+    parent.mkdir(mode=0o755)
+    parent.chmod(0o755)
+    assert _basetemp_for_rank(monkeypatch, tmp_path).parent == parent
+    assert parent.stat().st_mode & 0o777 == 0o700