"Delicious beef is very expensive."
Translation:맛있는 소고기는 많이 비싸요.
Why does delicious have an identification particle when beef does too?
"-는" is a present tense adjectival ending attached to 있다, 없다, 계시다, or an active verb. It just happened to have the same spelling as the particle "는".
I am wondering if 많이 has the +이 particle or if it is just coincidentally spelt with that at the end.
kinda has, kinda has not. The adverb '많이' is derivered from the adjective '많다', which means 'numerous'.
It is very common for Korean adverbs to be based on adjectives with '이' somehow attached at the end. For example: 열심하다 -> 열심히 (eager, eagerly); 영원하다 -> 영원히 (eternal, eternally); 빠르다 -> 빨리 (quick, quickly); 없다 -> 없이 (nonexistent, without)
however, DO NOT confuse this '이' with '이/가' subject marker! And also don't try to coin your own adverbs by adding '이' to random adjectives, because it simply doesn't work 90% of the time.
Why are there 2 topic marker (는) in the same sentence. Shouldn't there only be 1 and the 소고기 ends with 가 and not 는?
First of all, "-는" is a present tense adjectival ending attached to 있다, 없다, 계시다, or an active verb. It just happened to have the same spelling as the particle "는". Second, whilst there should be only one subject, there may be two or more topics.
In this sentence, it's too simple for there to be a difference between 가/이 and 은/는. You were probably marked wrong because you spelled 비싸요 wrong, or because you used 가장 비싸다, which translates to "the most expensive"
Your sentence would be something like "As for deliciousness, beef is the most expensive." That's because using the topic marking particle after 맛있다 indicates that you are emphasizing the deliciousness of what you're about to talk about. "맛있는 소고기는 가장 빘아요" would be, "Delicious beef is the most expensive."
For distinguishing between 이/가 and 은/는, here is a video I really enjoy about this topic/subject (sorry for the pun!): https://www.youtube.com/watch?v=6JSJ9yEWy1I
"As for deliciousness, beef is the most expensive." would be "맛은 소고기가 가장 비싸요." The "-는" after "맛있-" is not a particle.