sql, return all the rows that certain field value are not unique, how?

[searain]

tb1 has field1

and the table has the values as

1
2
3
3
4
5
6
7
7
8

I want to select all the rows that the field1 value is not unique. in another words, i want to return be

3
3
7
7

Thanks!

[Lars_Berg]

select field1 
from t
where field1 in
  (select field1
      from t
     group by field1
    having count(*) > 1)