If you play {snap caster Mage} and target a card that already has flashback, does it get another flashback choice? (A cheaper one)
Yes
I think the same effect that {Varolz, the Scar-Striped} generates will occur. If you target a card with a scavenge cost, it now has two instances of scavenge. The actual scavenge cost and its mana cost scavenge that is made by varol's ability.
For example: {dreg mangler} has a scavenge cost of {3}{B}{G}but with varolz on the board it can be scavenged for {1}{B}{G}
Same with a {Snapcaster Mage}. The card you target, if it already has a flashback cost, it now has two instances of flashback that can be paid. It's printed flashback cost on the card, and the flash back cost that snap generates.
For example: {Bump in the Night} has a flash back cost of {5}{R} but when targeted by snappy it can be cast for {B}