Trying to design a user interface screen for a part of the system where users will be able to select companies from a drop-down. The drop-down list is potentially very long. However, if the company is not there, they are allowed to add it in free text.

The important thing about this screen is that if the company is in the drop down list, it's critical that the user finds it and selects it. If they add it again, it could cause serious problems with the workflow.

What we've considered:

• A searchable (i.e. type in the box to get autocomplete suggestions) drop-down, with an "add new" option at the bottom of the list which they can't get by typing. That way users have to scroll all the way down to add something.

• A check-box toggle between a drop-down and a free-text entry, with a possible warning popup when they hit the checkbox.

Are there any other smoother, cleverer or more secure ways to try and ensure a user tries to find and select something already on the list before adding?