Yes, a good implementation should store just the closed variables and a reference to the code - there shouldn't be any code duplication.
It thus depends on how many local variables are being closed over. Note that in some implementations (although not in arc2c, and by inference not in SNAP) a closure is just two pointers: a reference to an environment and a reference to the code. Of course each 'let form and function would create a new environment though, and this style is not so often used because lookup of closed variables can require indirection.
