Fix SetLayout to error with invalid int list input

qiskit/transpiler/__init__.py

@@ -1248,6 +1248,7 @@
 .. autoexception:: CouplingError
 .. autoexception:: LayoutError
 .. autoexception:: CircuitTooWideForTarget
+.. autoexception:: InvalidLayoutError
 
 """
 
@@ -1266,6 +1267,7 @@
     CouplingError,
     LayoutError,
     CircuitTooWideForTarget,
+    InvalidLayoutError,
 )
 from .basepasses import AnalysisPass, TransformationPass
 from .coupling import CouplingMap

qiskit/transpiler/exceptions.py

@@ -53,3 +53,7 @@ def __str__(self):
 
 class CircuitTooWideForTarget(TranspilerError):
     """Error raised if the circuit is too wide for the target."""
+
+
+class InvalidLayoutError(TranspilerError):
+    """Error raised when a user provided layout is invalid."""

qiskit/transpiler/passes/layout/set_layout.py

@@ -11,7 +11,8 @@
 # that they have been altered from the originals.
 
 """Set the ``layout`` property to the given layout."""
-from qiskit.transpiler import Layout, TranspilerError
+from qiskit.transpiler import Layout
+from qiskit.transpiler.exceptions import InvalidLayoutError
 from qiskit.transpiler.basepasses import AnalysisPass
 
 
@@ -47,17 +48,21 @@ def run(self, dag):
         """
         if isinstance(self.layout, list):
             if len(self.layout) != len(dag.qubits):
-                raise TranspilerError(
+                raise InvalidLayoutError(
                     "The length of the layout is different than the size of the "
                     f"circuit: {len(self.layout)} <> {len(dag.qubits)}"
                 )
+            if len(set(self.layout)) != len(self.layout):
+                raise InvalidLayoutError(
+                    f"The provided layout {self.layout} contains duplicate qubits"
+                )
             layout = Layout({phys: dag.qubits[i] for i, phys in enumerate(self.layout)})
         elif isinstance(self.layout, Layout):
             layout = self.layout.copy()
         elif self.layout is None:
             layout = None
         else:
-            raise TranspilerError(
+            raise InvalidLayoutError(
                 f"SetLayout was intialized with the layout type: {type(self.layout)}"
             )
         self.property_set["layout"] = layout

releasenotes/notes/add-invalid-layout-error-d0d64086748d4b54.yaml

@@ -0,0 +1,13 @@
+---
+features:
+  - |
+    Added a new exception class :class:`.InvalidLayoutError` that is a :class:`.TranspilerError`
+    subclass which is raised when a user provided layout is invalid (mismatched size, duplicate
+    qubits, etc).
+fixes:
+  - |
+    Fixed an issue with the :class:`.SetLayout` transpiler pass where an invalid integer list input
+    that contained duplicate entries which would result in an invalid :class:`.Layout` being
+    generated and subsequent transpiler passes would fail with a cryptic error. This is now caught
+    when :meth:`.SetLayout.run` is called an :class:`.InvalidLayoutError` error will be raised
+    indicating there are duplicate entries in the integer list.

test/python/transpiler/test_setlayout.py

@@ -109,6 +109,13 @@ def test_raise_when_layout_len_does_not_match(self):
         with self.assertRaises(TranspilerError):
             pass_manager.run(circuit)
 
+    def test_raise_if_int_list_layout_contains_duplicates(self):
+        """Test the error is raised if the specified intlist contains duplicates"""
+        circuit = QuantumCircuit(4)
+        layout_pass = SetLayout([0, 1, 1, 2])
+        with self.assertRaises(TranspilerError):
+            layout_pass(circuit)
+
 
 if __name__ == "__main__":
     unittest.main()