Duncan might decide to "Rock 'n' Roll" and act rudely and obstinate, however Rusty had to rescue him after his accident. When he was first introduced in the fourth season, he was often stubborn.
Since the sixth season, Duncan is quite often bossy to the other engines about getting their work done quickly and thinks he knows better than them. Duncan still seems to ignore Rusty's warnings and advice until he lands himself into trouble. Also, Duncan tries to do the jobs which he thinks suits him well and he is quite competitive with James.