Rerun a previously entered command again in a different context.

First run, count all filesystem objects under the /tmp path:

user@host:~$ find /tmp | wc -l
1376

Rerun the same command, but this time in the context of the time command in order to measure how long it takes to complete:

user@host:~$ time !!
time find /tmp/ | wc -l
1376

real	0m0.008s
user	0m0.000s
sys	0m0.004s