interaction design - Usability of autocomplete overwriting user input

We have a web application for professional users, who have to register with their organisation and department. And of course there are tons of ways to write out the name of the same department. But because our application is open to everybody in that profession, we cannot use an existing list of departments (we would need a list of all departments in the world!)

Currently, users have a free text field with autocomplete suggestions for the department when registering. Nevertheless, our users frequently register an existing department a second time instead choosing their department's name from the autocomplete list.

We see three possible options here:

1. Leave it as it is, and manually merge double departments later


2. Use a dropdown for the organisation selection, with a "create new department" button for those whose department is not there.


3. Change the behavior of the autocomplete field. Once the user clicks outside of the field, the content gets replaced with the first autocomplete suggestion, whatever it was. The way for a user to avoid this is to first hover with the mouse cursor over the autocomplete suggestions but not select any, then click outside the autocomplete field or its list of suggestions. This way, the user always gets registered with the department whose name best matches the string he entered.

Which solution (of those above, or a different one) is likely to be better UX, given that we don't have the resources to A/B test.