Dec 17 (Reuters) - Canada"s main stock index opened lower on Friday, extending losses for the second session as energy stocks weighed, with concerns around surging Omicron cases around the world keeping sentiment in check. At 9:30 a.m. ET (14:30 GMT), the Toronto Stock Exchange"s S&P/TSX composite index (.GSPTSE) was down 29.67 points, or 0.14%, at 20,710.11.