Possible meanings are 1) Isaiah is saying two different things about them. Alternate translation: "Nation of Israel, you sinners" or 2) he is saying only one thing about them. Alternate translation: "Nation of sinners"
Something very heavy that a person might carry is a metaphor for their many sins. Alternate translation: "their sin is like a heavy bag on their shoulders that makes it hard for them to walk" (See: [[rc://en/ta/man/translate/figs-metaphor]])
The word "offspring" is a metaphor for people who do what others have done. Alternate translation: "people who do the same evil they see others doing" (See: [[rc://en/ta/man/translate/figs-metaphor]])