Try this:
import pandas
df = pandas.DataFrame({'col':['bbb','aaa','aaa','ccc']})
print df
print '-------------'
df2 = df.drop_duplicates()
df2['ranked'] = df2.rank(method='min')
print df.merge(df2, on='col')
Output:
col
0 bbb
1 aaa
2 aaa
3 ccc
-------------
col ranked
0 bbb 2
1 aaa 1
2 aaa 1
3 ccc 3