How would I make it re-roll if its taken?

You'll want to use loops basically

local hi = math.random(1,2)

while hi ~= 2 do -- while hi is not equal to 2 loop
    wait()
    hi = math.random(1,2)
end

that will work if you only want 2 choices if you want more something like this will work

local taken = {} -- store all the used values
local hi = math.random(1,5) -- final unused value
local foundOne = false -- keep while loop going
while foundOne == false do
    wait()
    foundOne = true -- make true since it assumes it's not taken until proven wrong
    local hi = math.random(1,5) -- change hi to new value
    for i = 1,#taken do -- loop through all used values
        if taken[i] == hi then -- value is taken
            foundOne = false -- continue loop
        end
    end
end
table.insert(taken,hi) -- add value to used list