Formatting multimap list to another format

So I've been working on HashMaps lately and found that it's not an ideal solution. When looking online I found multimaps to be a possible option. However it's not in the correct format. Multimap<Long, Integer> response = [login to view URL](); Response = {1=[2140001792, 2140001793]} However I want it to be like: {1=2140001792, 1=2140001792} I realised after asking a previous question HashMaps don't allow duplicate keys. Therefore with that, is there any possibility for me to change the format in either the back end or the AngularJS front end? Any help would be appreciated. Thank you.