interface and a class. name clash: same erasure, yet neither overrides other

Your GenericQueue is implementing the raw interface IGenericQueue, so its T is different than the T in IGenericQueue. Add the <T> in the implements clause:

public class GenericQueue<T extends Comparable> implements IGenericQueue<T> {
//                                                                      ^^^

so you are implementing the generic interface with the same T.