How do you limit an employee to selecting only one option within a plan?

Summary:

We set up a couple new plans this year, and it is allowing employee to select more than one option within a plan

Content (required):

We set up a couple new plans this year, and it is allowing employee to select more than one option within a plan. It ends in an error, which goes away when they select 1 option, but we can't figure out why it is allowing the selection of more than one option.

Example:

Plan: Vision

Options: Default = Waive and the employee selects EE Only, the system is not deselecting Waive as it does with other plans.