Hey guys - one thing I'll add before going further on that is check the table the dropdown is going to. Make sure the values are unique.
Since you are using the API, I could see you doing something like creating a new table, populating it with all the values and creating a dropdown in another table that references this new table. If you ran the "populating it with all values" proceedure a second time it could have duplicated the values.
If that's not the cause, then as Kris said, let's get a look in the database to see if there is corruption.