How can multiple image of docker point to the same id?

Such as ubuntu

ubuntu                saucy               9f676bd305a4        5 weeks ago         178 MB
 ubuntu                13.10               9f676bd305a4        5 weeks ago         178 MB
 ubuntu                raring              eb601b8965b8        5 weeks ago         166.5 MB
 ubuntu                13.04               eb601b8965b8        5 weeks ago         166.5 MB
 ubuntu                quantal             5ac751e8d623        5 weeks ago         161 MB
 ubuntu                12.10               5ac751e8d623        5 weeks ago         161 MB
 ubuntu                10.04               9cc9ea5ea540        5 weeks ago         180.8 MB
 ubuntu                lucid               9cc9ea5ea540        5 weeks ago         180.8 MB
 ubuntu                12.04               9cd978db300e        5 weeks ago         204.4 MB
 ubuntu                latest              9cd978db300e        5 weeks ago         204.4 MB
 ubuntu                precise             9cd978db300e        5 weeks ago         204.4 MB

For example, 12.04, precision and latest all point to 9cd978db300e.

If you commit yourself, how can you achieve this effect? I want to point the original latest tag to the latest tag every time I submit a new tag.