Need help with variables tracking progress

Hi, I am currently trying to create something which will track when the learners have visited a page. I have done this using a variable called 'Progress', once a learner has clicked the next button on any of the pages it will add 20 to the value of the variable. However if the learner goes back and then presses a next button more than once it will add more to the value, which sends the progress to higher than 100% which shouldn't happen. 

So is there a way of setting it so that 20 is only added to the value once and after that the learner can still click the next button but no value would be added.

Any help would be really appreciated! 

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