diff --git a/masque/ports.py b/masque/ports.py
index 8d7b6a6..3a695fb 100644
--- a/masque/ports.py
+++ b/masque/ports.py
@@ -510,10 +510,19 @@ class PortList(metaclass=ABCMeta):
         if missing_invals:
             raise PortError(f'`map_in` values not present in other device: {missing_invals}')
 
+        map_in_counts = Counter(map_in.values())
+        conflicts_in = {kk for kk, vv in map_in_counts.items() if vv > 1}
+        if conflicts_in:
+            raise PortError(f'Duplicate values in `map_in`: {conflicts_in}')
+
         missing_outkeys = set(map_out.keys()) - other
         if missing_outkeys:
             raise PortError(f'`map_out` keys not present in other device: {missing_outkeys}')
 
+        connected_outkeys = set(map_out.keys()) & set(map_in.values())
+        if connected_outkeys:
+            raise PortError(f'`map_out` keys conflict with connected ports: {connected_outkeys}')
+
         orig_remaining = set(self.ports.keys()) - set(map_in.keys())
         other_remaining = other - set(map_out.keys()) - set(map_in.values())
         mapped_vals = set(map_out.values())
diff --git a/masque/test/test_ports.py b/masque/test/test_ports.py
index 0f60809..4e7d097 100644
--- a/masque/test/test_ports.py
+++ b/masque/test/test_ports.py
@@ -4,6 +4,7 @@ from numpy import pi
 
 from ..ports import Port, PortList
 from ..error import PortError
+from ..pattern import Pattern
 
 
 def test_port_init() -> None:
@@ -227,3 +228,32 @@ def test_port_list_plugged_mismatch() -> None:
     pl = MyPorts()
     with pytest.raises(PortError):
         pl.plugged({"A": "B"})
+
+
+def test_port_list_check_ports_duplicate_map_in_values_raise() -> None:
+    class MyPorts(PortList):
+        def __init__(self) -> None:
+            self._ports = {"A": Port((0, 0), 0), "B": Port((0, 0), 0)}
+
+        @property
+        def ports(self) -> dict[str, Port]:
+            return self._ports
+
+        @ports.setter
+        def ports(self, val: dict[str, Port]) -> None:
+            self._ports = val
+
+    pl = MyPorts()
+    with pytest.raises(PortError, match="Duplicate values in `map_in`"):
+        pl.check_ports({"X", "Y"}, map_in={"A": "X", "B": "X"})
+    assert set(pl.ports) == {"A", "B"}
+
+
+def test_pattern_plug_rejects_map_out_on_connected_ports_atomically() -> None:
+    host = Pattern(ports={"A": Port((0, 0), 0)})
+    other = Pattern(ports={"X": Port((0, 0), pi), "Y": Port((5, 0), 0)})
+
+    with pytest.raises(PortError, match="`map_out` keys conflict with connected ports"):
+        host.plug(other, {"A": "X"}, map_out={"X": "renamed", "Y": "out"}, append=True)
+
+    assert set(host.ports) == {"A"}