diff --git a/masque/ports.py b/masque/ports.py
index ff3a0e3..3a67003 100644
--- a/masque/ports.py
+++ b/masque/ports.py
@@ -368,6 +368,8 @@ class PortList(metaclass=ABCMeta):
         Returns:
             self
         """
+        if names[0] == names[1]:
+            raise PortError(f'Port names must be distinct: {names[0]!r}')
         new_ports = {
             names[0]: Port(offset, rotation=rotation, ptype=ptype),
             names[1]: Port(offset, rotation=rotation + pi, ptype=ptype),
diff --git a/masque/test/test_ports.py b/masque/test/test_ports.py
index 14dc982..0291a1c 100644
--- a/masque/test/test_ports.py
+++ b/masque/test/test_ports.py
@@ -89,6 +89,25 @@ def test_port_list_rename_missing_port_raises() -> None:
     assert set(pl.ports) == {"A"}
 
 
+def test_port_list_add_port_pair_requires_distinct_names() -> None:
+    class MyPorts(PortList):
+        def __init__(self) -> None:
+            self._ports: dict[str, Port] = {}
+
+        @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="Port names must be distinct"):
+        pl.add_port_pair(names=("A", "A"))
+    assert not pl.ports
+
+
 def test_port_list_plugged() -> None:
     class MyPorts(PortList):
         def __init__(self) -> None:
@@ -126,6 +145,29 @@ def test_port_list_plugged_empty_raises() -> None:
     assert set(pl.ports) == {"A", "B"}
 
 
+def test_port_list_plugged_missing_port_raises() -> None:
+    class MyPorts(PortList):
+        def __init__(self) -> None:
+            self._ports = {"A": Port((10, 10), 0), "B": Port((10, 10), pi)}
+
+        @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="Connection source ports were not found"):
+        pl.plugged({"missing": "B"})
+    assert set(pl.ports) == {"A", "B"}
+
+    with pytest.raises(PortError, match="Connection destination ports were not found"):
+        pl.plugged({"A": "missing"})
+    assert set(pl.ports) == {"A", "B"}
+
+
 def test_port_list_plugged_mismatch() -> None:
     class MyPorts(PortList):
         def __init__(self) -> None: