Complex sentence: Along with Coles, Woolworths form a near duopoly of Australian supermarkets, together accounting for about 80% of the Australian market, however the Australian expansion of discount supermarket chain Aldi has seriously challenged this duopoly in recent years.
Split into simpler sentences: Along with Coles, Woolworths form a near duopoly of Australian supermarkets, together accounting for about 80% of the Australian market. Although the Australian expansion of Aldi has seriously challenged this duopoly in recent years.