Merge #10503: Use REJECT_DUPLICATE for already known and conflicted txn

Summary:
PR10503 backport https://github.com/bitcoin/bitcoin/pull/10503/files
d9bec888f Use REJECT_DUPLICATE for already known and conflicted txn (Pieter Wuille)

(nb one change was already done in D1734)

Test Plan:
make check
test_runner.py

Reviewers: jasonbcox, Fabien, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3078