Allowing Duplicate Keys in Java Collections
Do it yourself Multi Value Map with Java 8
How to Store Duplicate Keys in a Map in Java?
Use Duplicate Keys in a Map with Multimap from Google Guava
Why HashMap not allowing duplicate keys? How it is stored?

org.apache.commons.collections4.SetValuedMap
org.apache.commons.collections4.multimap.HashSetValuedHashMap
com.google.common.collect.HashMultimap
org.eclipse.collections.api.multimap.set.MutableSetMultimap
org.springframework.util.MultiValueMap