local s = {1, 2, 2, 3, 4, 4, 5}

-- There is no output as 'prev' is created anew each time
-- around the loop and set implicitly to nil.
for i = 1, #s do
    local curr = s[i]
    local prev
    if i > 1 and curr == prev then print(i) end
    prev = curr
end

-- Now 'prev' is created only once and reassigned
-- each time around the loop producing the desired output.
local prev
for i = 1, #s do
    local curr = s[i]
    if i > 1 and curr == prev then print(i) end
    prev = curr
end