This is due to your order of events.
In event 2, Level is equal to 1. One gets added to Level. Level now equals 2.
In the next event, Level equals 2, because you changed it already in the previous event. So it meets the condition for "Level equal to 2" and the event triggers, which adds 1 to Level. Now it equals three.
In the next event the same thing happens. And so on down the list until you hit six.
So, you could do this a few different ways. The first is you could add another criteria to your conditions to keep them from triggering all in the same tick. I would make another variable called "switch" or something like that, that turns On when you click the button, and turns Off when you level up. That way you can check Switch in your conditions, and if it is Off, then the level-up action won't run. Like this:
+On button clicked
-> Set Switch On
+ Sprite Level = 1
+ Switch = On
-> Level up
-> Set Switch Off
+ Sprite Level = 2
+ Switch = On
-> Level up
-> Set Switch Off
Etc.
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The second way you could do this is to reverse the order of your level code. So it would look like this:
[code:puxsx5mi]
+ On button clicked
+ Level = 5
-> Set level to 6
+ Level = 4
-> Set level to 5
Etc.
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And finally, since it seems you are doing a really simple linear calculation to increase your levels, you could simply do this:
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+ On button clicked
-> Add 1 to level
-> Multiply Attack x 2
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And leave it at that.