Breakdown of Mi hermana quiere comprar un marco de madera y un jarrón sencillo para el salón nuevo.
Questions & Answers about Mi hermana quiere comprar un marco de madera y un jarrón sencillo para el salón nuevo.
Because Spanish normally uses an article before each singular countable noun when you are listing separate items.
So:
This makes it clear that these are two different objects.
If you left out the second un, the sentence could sound less natural in this context. Repeating it is the standard and clearest option.