1. Create an "admin" user in JIRA if there isn't already
2. Create a "testadmin" user in JIRA
3. Create a custom Multi User Picker field & add it to the default screen
4. Create a project if there isn't an existing one
5. Create an issue
6. In the multi user picker field, type "testadmin, "
7. Start typing "admin", then pick it from the autocomplete drop-down

Result: An error appears saying, "Oops! You have already entered the value admin" and removes the "admin" value

The JavaScript duplicate detection appears to be matching against the end of previous values, rather than the entire value.