Entered my Ontario eligible tuition but this amount has been doubled on Schedule ON(S11) line 5914. Why?

The only way it gets doubles is if you have used the CRA auto-fill- which would of added it already.

It has to do with the entry for the portion of tuition fees applicable to studies before Sept 5 - when I enter the amount (full tuition as graduated in April 2017) it doubles the value

How did you fix this?

Figured out I was entering the amount twice - once on the screen where it asks for values that you do not have slips for and then I was entering it again on the screen from my actual slip amounts.