Q:

Article “the“ for superlative

Which one is correct:
“XYZ is the most famous and the biggest concert in Australia.“ or 
“XYZ is the most famous and biggest concert in Australia.“?

A:

Welcome to ELL!
The first is strictly correct.
It is the word most that causes the problem, I think. The second sentence is slightly awkward as it implies a double superlative: most biggest. Double superlatives are regarded as non-standard nowadays. [Shakespeare often used them however.] 
The repeat of the in “the most famous and the biggest" avoids the problem, but it might be even better and more idiomatic to say "the biggest and most famous".

