Hi there, the second 'which' can be omitted, but it can also be left in the sentence. There is a difference between the 2 versions of 'which'. The first 'which' just gives some extra information about the street. If you take this part out of the overall sentence it doesn't really affect the meaning of the sentence in any way. With these, you need to always include 'which', and there should be a comma before 'which'. The second which is necessary information about the shop. If you take this part out of the sentence it would mean that the sentence has lost some valuable information. With these you can leave 'which' out if it comes immediately before the subject of the sentence (usually a noun, in this case 'Toma') If we look at 2 other sentences it will make things clearer: 1st which: 'We went to the shop, which he liked' - We went to the shop. He liked the fact that we went to the shop. This which gives some extra information about the whole experience. Here we must use a comma before which and we must always include 'which'. 2nd which: 'We went to the shop which he liked' - We ONLY went to the shop which he liked. We didn't go to any other shops. Here the which part is very important because it contributes something to the meaning of the sentence. Here there should be no comma before which, and the which can be omitted in these sentences ONLY IF IT COMES BEFORE A NOUN OR PRONOUN. You cannot omit this which if there is a verb straight after which: 'We went to the shop which was good' So here is a bit of a summary 1st which - just extra information, can take it our of the sentence. Must use commas and must use which 2nd which - information which changes the meaning of the sentence and the noun before it. No commas and which can be omitted if it comes before a noun or pronoun (subject of the sentence) I hope this helps. Here's a link to a more detailed explanation and more examples: https://en.oxforddictionaries.com/grammar/relative-clauses

Yes you can make what are called elliptical constructions. They are very common. So, "At the end of the street [, which was] crowded with shoppers, there was a building [which] Toma had not noticed before." Either way is fine grammatically. But when in doubt follow the Latinate (Spanish) pattern.