Also note that you can now right-click a speed dial and choose edit to edit its thumbnail. It only works for non-default ones. But, if you want to change a default one, move it to the trash, goto the URL opera://bookmarks, empty the trash, and then add the site as a speed dial yourself. That one you'll be able to edit.
If you need a default thumbnail that Opera no longer uses, some of them are available here.
@iamsilent121 oh wow thanks, it was driving me nuts and last time I checked I couldn't find the setting responsible, the weird thing is that it seem to only autocomplete if you type fast enough, which is even more stupid, if I'm typing that fast I'm likely to typo duplicated letters even if I anticipate the autocomplete. Glad to have finally found the way to turn it off