It's for (dis)advantage
You're playing D&D 5e, there is the rule for (dis)advantage there:
you roll a second d20 when you make the roll. Use the higher of the two rolls if you have advantage, and use the lower roll if you have disadvantage
The charsheet you're using has the special feature for this rule:
With the Roll20 5th Edition OGL Sheet, the default sheet settings has Advantage set to always rolled for convenience purposes.
Your particular character has the "always roll advantage" option turned on:
That means every time you make a roll the system actually makes two rolls. You take higher result if there was an advantage, lower result if there was a disadvantage or a particular result (the left one, for instance) every time there was neither advantage nor disadvantage. That helps all the participants to save a little time.
If you don't want these double rolls, you can switch the option to "never roll advantage" (or ask your DM to do that). There is also "query advantage" option, when the system asks you if you roll with (dis)advantage every time.
Assassinate damage is another thing
The lower set of numbers is due to the Assassinate feature — the first number is standard damage, the second is the automatic critical damage if the target is surprised (thanks @Journer for this catch).