SQL & PL/SQL :: Rank Result With Respect To Order

I have a SQL statement that return the following result.

col1
----
a
a
b
b
a
c
c
c
c

What i need is to eliminate the duplication and return the result as the following.

col1
----
a
b
a
c

Using group by or distinct will eliminate the third row "a" and will not return it ordered.My idea is to generate another column that rank the result as following.

col1rank
------------
a1
a1
b2
b2
a3
c4
c4
c4
c4

so using the following SQL query will return the result as needed

select col1, rank
from T
group by col1, rank
order by rank;

col1rank
------------
a1
b2
a3
c4