Translation:I want to buy a bag of rice.
Yeah, Mandarin seems to have a lot of double words for things. 米饭 is also a word for rice.
I don’t know for sure but my experience is that 米 tends to be when you’re dealing with the grain (or ingredient) and 饭 when you’re eating it (cooked). 米 always seems to mean rice whereas 饭 can be used to just mean food (generally prepared/cooked food).