According to https://proofreadmydocument.com.au/writing-tips/spelling-tips-the-doubling-up-rule/ and https://www.macquariedictionary.com.au/resources/view/resource/20/, we should spell the past participle of "develop" as "developed", and the present particple as "developing" (mult-syllable, unstressed final syllable of the stem). Goolge prefers it, too. Still, it provides us with over 23800 references for
site:.au "developped"
Are they all wrong?
As for Bing,
site:.au "developed"
and
site:.au "developped"
have approximately the same counts.