How are variable totals affected when a learner changes their selection?

Hey heroes, I've got a question about setting up variable triggers. If you're familiar with this, thanks in advance for saving me the trial and error process.

I'm setting up a slide where a learner chooses one of 3 options for each learning component in a list of 8, gauging how confident they feel about each one.  I'm thinking of setting up text with buttons in 3 columns - very confident, somewhat, and not yet.  I'd like each button to add a value to a number variable so the learner ultimately gets a value summary.  So the variable would increase by 1 point for somewhat, 2 points for very, and none for not yet.  They would then get a total showing "you've indicated you feel X% confident about the material".

My Question:

A learner might waffle about how confident they feel and change their selection. How can this be handled so that only one selection triggers an addition to the variable?

Cheers thanks friends!