Show option is both required and previously used in dropdown list

I realize this is probably not the best way to handle the problem, but I need to shoehorn in a solution to a previously existing design - and a dropdown list was used.

In this situation there are a number of fields the user needs to add from a dropdown list to a table. Some are fields required, some are not... each can be used only once.

The request is to indicate which items in the drowdown are required and indicate if they have been previously added.

What is the best option for this?

My first thought is to use a red asterisk next to required item and change it to a checkmark when previously added as such:

--------------
| dropdown ▼ |
|------------|
| * option 1 |
| * option 2 |
| ✓ option 3 |
|   option 4 |
| ✓ option 5 |
| * option 6 |
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