If two classes share a lot of each other's methods then there is a lot of unnecessary code duplication. This makes it difficult to make changes in future and makes the code harder to read.

If a duplicate class has been included by mistake then remove it. Otherwise consider making a common superclass for both classes or even making one of the classes a superclass of the other.

Elmar Juergens, Florian Deissenboeck, Benjamin Hummel and Stefan Wagner. Do Code Clones Matter?. 2009.