If an user copies an URL "A" and shortens it using alias "X", now if another user copies URL "B" and gives it the same alias name "X". Then the newly shortened URL still points to the URL "A" and the pointer to URL "B" is lost i.e. URL "B" is not shortened although it shows on the website as URL "B" is shortened.
Yes that happens pretty often because that was a patch which worked correctly only in certain conditions. For now i have pulled down the site to code the workaround. If you find any you are welcome to file a PR.
If an user copies an URL "A" and shortens it using alias "X", now if another user copies URL "B" and gives it the same alias name "X". Then the newly shortened URL still points to the URL "A" and the pointer to URL "B" is lost i.e. URL "B" is not shortened although it shows on the website as URL "B" is shortened.