Guns N' Roses Bassist Duff McKagan -- 'Sweet Child O' Mine' WAS Jacked ... JK???

The "Sweet Child O' Mine" plot just thickened ... based on ex-Guns N' Roses bassist Duff McKagan's response to allegations the band's mega-hit was ripped off from an Australian group.

Duff was in NYC when we asked him about the big shocker -- the eerie similarity between 'Sweet Child' and an Australian hit called "Unpublished Critics" ... which came out SIX years before the GnR classic.

Watch ... our photog clearly expects Duff to quickly shoot down the claim -- but instead he says the Aussie group, Australian Crawl, deserves credit ... saying, "they did" write it!