When I roll in Roll20 (with my crossbow as an example) I get output, like below, in the game log. Why are there two numbers?
In the below example, there is the number 8 and the number 25.
The question also pertains to the Assassinate ability roll below the crossbow one.
I'm using the default character sheet for D&D 5e – 5th Edition OGL by Roll20.
It's for (dis)advantage
You're playing D&D 5e, there is the rule for (dis)advantage there:
The charsheet you're using has the special feature for this rule:
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).