You may add DISTINCT in the SELECT statement. This will remove the duplicates.
select DISTINCT c1.MANDT, c1.BUKRS, c1.WAERS, c1.KTOPL, c1.GSBER, c1.VRGNG,
c1.PERNR, c1.EBELP, c1.MEINB, c1.KSTAR, c1.MATNR, c1.PBUKRS, c1.FKBER,
c1.KOKRS, c1.BELNR, c1.BUZEI, c1.SGTXT, c1.VERSN, c1.MEINH, c1.PFKBER,
c1.REFBN, c1.BUDAT, c1.CPUTM, c1.CPUDT, c1.BLDAT, c1.KOSTL, C1.SMEV,
c1.SMEF, c1.SMEG, c1.SMEG1, c1.SWF, c1.SWG, c1.LSTAR, c1.PRCTR, c1.fiscper as FISCPER_ACT,
C2.fiscvar, c2.fiscper, c2.kokrs as kokrs_p, c2.kostl as kostl_p,
c2.vtype, c2.versn as versn_p, c2.kstar as kstar_p, c2.seknz, c2.valutyp,
c2.meastype, c2.swg as swg_p, c2.swf as swf_p, c2.swv as swv_p, c2.waers as waers_p,
c2.vrgng as vrgng_p, c2.vtdetail, c2.vtstat, c2.lstar as lstar_p
from "_SYS_BIC"."dev/CV_CCA" as C1
full outer join "_SYS_BIC"."dev/CV_CCL_PLAN" as C2
on c1.mandt = c2.mandt and
c1.KOSTL = c2.KOSTL and
c1.KOKRS = c2.KOKRS and
c1.kstar = c2.kstar and
c1.FISCPER = c2.fiscper ;
Also please check what the business requirement is for the FULL OUTER JOIN. By definition, this join will produce all combinations and generally has the potential to create lot of duplicate records.