Dobrando o lance anterior (Exception) - Exercício 5

Após implementar os trechos abaixo (primeira etapa do exercício 5):

LeilaoTest.java

    @Test
    public void deveDobrarOUltimoLanceDados() {
        Leilao leilao = new Leilao("Macbook Pro 15");
        Usuario steveJobs = new Usuario("Steve Jobs");
        Usuario billGates = new Usuario("Bill Gates");

        leilao.propoe(new Lance(steveJobs, 2000));
        leilao.propoe(new Lance(billGates, 3000));
        leilao.dobraLance(steveJobs);

        assertEquals(4000.0, leilao.getLances().get(2).getValor(), 0.00001);
    }

Leilao.java

    public void dobraLance(Usuario usuario) {
        Lance ultimo = null;

        for(Lance lance : lances) {
            if(lance.getUsuario().equals(usuario))
                ultimo = lance;

            propoe(new Lance(usuario, ultimo.getValor()*2));           
        } 
    }

ocorre a seguinte exception:

java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:859)
    at java.util.ArrayList$Itr.next(ArrayList.java:831)
    at br.com.caelum.leilao.dominio.Leilao.dobraLance(Leilao.java:51)
    at br.com.caelum.leilao.dominio.LeilaoTest.deveDobrarOUltimoLanceDados(LeilaoTest.java:88)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)