Redis duplicate key issue when doing a SCAN

redis-scan-duplicate-scan.md

Here's how to reproduce a scenario when the same key may be returned during a continuous SCAN.
A detailed explanation: https://groups.google.com/d/msg/redis-db/2V5x4-MOOEk/jNDUY11AEZYJ

> flushdb

OK

> mset 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18

OK

> scan 0 count 3

1. "28"
2. 1. "6"
   2. "17"
   3. "5"

> scan 28 count 3

1. "6"
2. 1. "12"
   2. "10"
   3. "11"
   4. "15"

> scan 6 count 3

1. "29"
2. 1. "14"
   2. "7"
   3. "2" <----- dupe
   4. "3"

> del 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3

(integer) 16

> scan 29 count 3

1. "0"
2. 1. "2" <----- dupe
   2. "1"