for merging, warccat concat does the job.
for cleaning/dedup: using JWAT-Tools you can unpack a warc file
jwattools.sh unpack your-archive.warc.gz
the result are single files containing a warc record. then you can remove duplicates or unwanted things and reconstruct the warc just with cat
cat your-archive.warc.{1,6,10} >your-new-new.warc
update: https://github.com/tari/warcdedupe