I got it.
First, remove the bottom event that you say is causing the problem. It's easier for me to explain this from scratch:
Player->On collision with Dimentionator->Add 1 to CurrLevel, Save game to mysave
(on a separate event, not sub event) System->On save complete->Go to layout str(CurrLevel)
=== That's it ===
"Trigger" condition, that is, the ones with the small green arrow on the event icons, cannot be used twice in an event block, including sub events.
"Is overlapping" will continuously append 1 onto CurrLevel, and "Trigger Once" can't stop it.
Ideally "On collision with" should be used instead of "overlapping", but because "on save complete" is a trigger in itself, "On collision with" isn't available as an option and "overlapping" just won't stop adding onto CurrLevel. The solution above will work.