Selling heirlooms?

 I am having trouble figuring out how to keep one in storage. I have tried removing the heirloom case and Using the display cases using  only putting one Of the heirlooms I know I have duplicates of in my room and it still is telling me that I can’t sell because the duplicate is not in storage I’m not sure if this is a bug or if I’m doing something incorrect of the heirlooms I know I have duplicates of in my room and it still is telling me that I can’t sell because the duplicate is not in storage I’m not sure if this is a bug or if I’m doing something incorrect.  Your answer was helpful but if you know how to explain this second step that I am seeming to have a problem with and less again it’s a bug that would be super great!

thanks in advance for your help/advice