The fastest, type-agnostic way to copy a Redis key, as discussed in https://redislabs.com/blog/the-7th-principle-of-redis-we-optimize-for-joy

00_copy_key.lua

-- @desc:   The fastest, type-agnostic way to copy a Redis key
-- @usage:  redis-cli --eval copy_key.lua <source> <dest> , [NX]

local s = KEYS[1]
local d = KEYS[2]

if redis.call("EXISTS", d) == 1 then
  if type(ARGV[1]) == "string" and ARGV[1]:upper() == "NX" then
    return nil
  else
    redis.call("DEL", d)
  end
end

redis.call("RESTORE", d, 0, redis.call("DUMP", s))
return "OK"

01_create_big_list.txt

foo@bar:~$ redis-benchmark -r 10000000 -n 10000000 -P 1000 lpush L __rand_int__
====== lpush L __rand_int__ ======
  10000000 requests completed in 1.30 seconds
…
814929.50 requests per second

foo@bar:~$ redis-cli llen  L
(integer) 10030000

02_copy_list_with_popnpush.lua

-- @desc:   copies a list with POP and PUSH
-- @usage:  redis-cli --eval copy_list_with_popnpush.lua <source> <dest>

local s = KEYS[1]
local d = KEYS[2]
local l = redis.call("LLEN", s)
local i = tonumber(l)

while i > 0 do
  local v = redis.call("RPOPLPUSH", s, s)
  redis.call("LPUSH", d, v)
  i = i - 1
end

return l

03_time_popnpush.txt

foo@bar:~$ time redis-cli --eval copy_list_with_popnpush.lua L T
(integer) 10030000

real	0m23.579s
user	0m0.000s
sys	0m0.006s

04_copy_list_with_lrange.lua

-- @desc:   copies a list with LRANGE
-- @usage:  redis-cli --eval copy_list_with_lrange.lua <source> <dest>

local s = KEYS[1]
local d = KEYS[2]
local i = tonumber(redis.call("LLEN", s))
local j = 0

while j < i do
  local l = redis.call("LRANGE", s, j, j+99)
  redis.call("LPUSH", d, unpack(l))
  j = j + 100
end

05_time_lrange.txt

foo@bar:~$ redis-cli del T
(integer) 1
foo@bar:~$ time redis-cli --eval copy_list_with_lrange.lua L T
(integer) 10030000

real	0m11.148s
user	0m0.000s
sys	0m0.004s

06_time_sort.txt

foo@bar:~$ redis-cli del T
(integer) 1
foo@bar:~$ time redis-cli sort L by nosort store T
(integer) 10030000

real	0m2.390s
user	0m0.000s
sys	0m0.003s

07_time_copy.txt

foo@bar:~$ redis-cli del T
(integer) 1
foo@bar:~$ time redis-cli --eval copy_key.lua L T
"OK"

real	0m1.661s
user	0m0.000s
sys	0m0.007s

RESTORE's new REPLACE modifier could be used instead of the NX-triggered DEL, but the latter is employed in this script to ensure compatibility with older versions.

For Sorted Sets, a self-union is actually faster. Source: http://stackoverflow.com/questions/32911604/intersection-of-two-or-more-sorted-sets/32980086#32980086