query assistance

Is there a straight forward way to pull out duplicates in a particular field given a value in another field?
For example, I have a table that lists users and study names associated with those users. Each user can have one or more study names. My goal is to determine if any of these people have duplicate study names. There are duplicated study names across the system and that is ok. I just want to see if any users have duplicate study names among their studies.
My table looks like this:

                                   Table "public.study"
    Column    |            Type             |                Modifiers                
--------------+-----------------------------+------------------------------------------
 sty_pk       | integer                     | not null default nextval('pk_seq'::text)
 study_name   | character varying(128)      | not null
 start_date   | timestamp without time zone |
 sty_comments | text                        |
 created_by   | integer                     |
Indexes: study_pkey primary key btree (sty_pk)

I am concerened with study_name. The created_by field tells me who owns it.
Thanks
Jodi