Number one in the stink parade is duplicated code. If you see the same code structure in more than one place, you can be sure that your program will be better if you find a way to unify them.
The simplest duplicated code problem is when you have the same expression in two methods of the same class. Then all you have to do is Extract Method and invoke the code from both places.
Another common duplication problem is when you have the same expression in two sibling subclasses. You can eliminate this duplication by using Extract Method in both classes then Pull Up Field. If the code is similar but not the same, you need to use Extract Method to separate the similar bits from the different bits. You may then find you can use Form Template Method. If the methods do the same thing with a different algorithm, you can choose the clearer of the two algorithms and use Substitute Algorithm.
If you have duplicated code in two unrelated classes, consider using Extract Class in one class and then use the new component in the other. Another possibility is that the method really belongs only in one of the classes and should be invoked by the other class or that the method belongs in a third class that should be referred to by both of the original classes. You have to decide where the method makes sense and ensure it is there and nowhere else.