No transaction in the mempool should ever be a coinbase.
Since mempoolDuplicate's backend is the chainstate coins view, it should
always contain the coins available.
This is a backport of core#23157 [7/8]
https://github.com/bitcoin/bitcoin/pull/23157/commits/ed6115f1eae0eb4669601106a9aaff078a2f3a74
Depends on D12172