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Take for example the case where Class A has a getSomething method and class B has a getSomething method and class C extends A and B. What would happen if someone called C.getSomething? There is no way to determine which method to call.
Interfaces basically just specify what methods a implementing class needs to contain. A class that implements multiple interfaces just means that class has to implement the methods from all those interfaces. Whci would not lead to any issues as described above.