Minor: add a unit test to verify that duplicated inputs are forbidden (conflict) and delete the TODO for it.

src/core/Transactions.kt

@@ -165,10 +165,6 @@ data class TimestampedWireTransaction(
  * A LedgerTransaction wraps the data needed to calculate one or more successor states from a set of input states.
  * It is the first step after extraction from a WireTransaction. The signatures at this point have been lined up
  * with the commands from the wire, and verified/looked up.
- *
- * Not used yet.
- *
- * TODO: When converting LedgerTransaction into TransactionForVerification, make sure to check for duped inputs.
  */
 data class LedgerTransaction(
     /** The input states which will be consumed/invalidated by the execution of this transaction. */

tests/core/TransactionGroupTests.kt

@@ -97,4 +97,25 @@ class TransactionGroupTests {
         }
         assertEquals(e.hash, ref.txhash)
     }
+
+    @Test
+    fun duplicatedInputs() {
+        // Check that a transaction cannot refer to the same input more than once.
+        transactionGroup {
+            roots {
+                transaction(A_THOUSAND_POUNDS label "£1000")
+            }
+
+            transaction {
+                input("£1000")
+                input("£1000")
+                output { A_THOUSAND_POUNDS.copy(amount = A_THOUSAND_POUNDS.amount * 2) }
+                arg(MINI_CORP_KEY) { Cash.Commands.Move }
+            }
+
+            assertFailsWith(TransactionConflictException::class) {
+                verify()
+            }
+        }
+    }
 }