Does this problem happen with the test project you posted? I agree with Allan, you are probably sending this event many times in your game. This can happen, for example, if you use "Is touching button" event instead of "On touched button".
I've implemented GA in many games and never had this issue.
You can try adding a counter (global variable), which increments every time you are sending the progression event, and display this counter in all layouts in your game. Also, check Realtime-> Live Feed in GA dashboard.