[commands] Improve error message when composing commands twice in same composition (#6091)

Also disallow deadline command from also appearing in other commands.

wpilibNewCommands/src/test/java/edu/wpi/first/wpilibj2/command/MultiCompositionTestBase.java

@@ -5,6 +5,7 @@
 package edu.wpi.first.wpilibj2.command;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.params.provider.Arguments.arguments;
 
 import edu.wpi.first.wpilibj2.command.Command.InterruptionBehavior;
@@ -112,4 +113,19 @@ abstract class MultiCompositionTestBase<T extends Command> extends SingleComposi
     var command = compose(command1, command2, command3);
     assertEquals(expected, command.runsWhenDisabled());
   }
+
+  static Stream<Arguments> composeDuplicates() {
+    Command a = new InstantCommand(() -> {});
+    Command b = new InstantCommand(() -> {});
+    return Stream.of(
+        arguments("AA", new Command[] {a, a}),
+        arguments("ABA", new Command[] {a, b, a}),
+        arguments("BAA", new Command[] {b, a, a}));
+  }
+
+  @MethodSource
+  @ParameterizedTest(name = "composeDuplicates[{index}]: {0}")
+  void composeDuplicates(@SuppressWarnings("unused") String name, Command[] commands) {
+    assertThrows(IllegalArgumentException.class, () -> compose(commands));
+  }
 }

wpilibNewCommands/src/test/java/edu/wpi/first/wpilibj2/command/ParallelDeadlineGroupTest.java

@@ -4,6 +4,7 @@
 
 package edu.wpi.first.wpilibj2.command;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
@@ -124,6 +125,21 @@ class ParallelDeadlineGroupTest extends MultiCompositionTestBase<ParallelDeadlin
         IllegalArgumentException.class, () -> new ParallelDeadlineGroup(command1, command2));
   }
 
+  @Test
+  void parallelDeadlineSetDeadlineToDeadlineTest() {
+    Command a = new InstantCommand(() -> {});
+    ParallelDeadlineGroup group = new ParallelDeadlineGroup(a);
+    assertDoesNotThrow(() -> group.setDeadline(a));
+  }
+
+  @Test
+  void parallelDeadlineSetDeadlineDuplicateTest() {
+    Command a = new InstantCommand(() -> {});
+    Command b = new InstantCommand(() -> {});
+    ParallelDeadlineGroup group = new ParallelDeadlineGroup(a, b);
+    assertThrows(IllegalArgumentException.class, () -> group.setDeadline(b));
+  }
+
   @Override
   public ParallelDeadlineGroup compose(Command... members) {
     return new ParallelDeadlineGroup(members[0], Arrays.copyOfRange(members, 1, members.length));