Benefits pl sql

I'm trying to make a query to get staff's benefit selection and their rate. However my query returning duplicate lines.

Can anyone help to see which filter or join I am missing? Below is my sql statement.

SELECT DISTINCT pl.PL_ID
, pl.NAME PL_NAME
, oipl.oipl_id
, OPT.OPT_ID
, OPT.NAME OPT_NAME
, rt.rt_val er_rate
from ben_pl_f pl,
ben_oipl_f oipl,
ben_opt_f opt,
ben_prtt_rt_val rt,
ben_prtt_enrt_rslt_f prtt
WHERE
--plan
    pl.effective_start_date <= '31-dec-2011'
    and pl.effective_end_date >= '01-jan-2011'