rpc: Remove submitblock invalid-duplicate precheck

ProcessNewBlock fails if an invalid duplicate block is passed in through
its call to AcceptBlock and AcceptBlockHeader. The failure in
AcceptBlockHeader makes AcceptBlock return early. This makes the
pre-check in submitblock redundant.

---

With the introduction of a mining ipc interface and the potential future
introduction of a kernel library API it becomes increasingly important
to offer common behaviour between them. An example of this is
ProcessNewBlock, which is used by ipc, rpc, net_processing and
(potentially) the kernel library. Having divergent behaviour on
suggested pre-checks and checks for these functions is confusing to both
developers and users and is a maintenance burden.

The rpc interface for ProcessNewBlock (submitblock) currently pre-checks
if the block has a coinbase transaction and whether it has been
processed before. While the current example binary for how to use the
kernel library, bitcoin-chainstate, imitates these checks, the other
interfaces do not.

Author: TheCharlatan

src/rpc/mining.cpp

@@ -1024,9 +1024,6 @@ static RPCHelpMan submitblock()
             if (pindex->IsValid(BLOCK_VALID_SCRIPTS)) {
                 return "duplicate";
             }
-            if (pindex->nStatus & BLOCK_FAILED_MASK) {
-                return "duplicate-invalid";
-            }
         }
     }
 

src/validation.cpp

@@ -4285,7 +4285,7 @@ bool ChainstateManager::AcceptBlockHeader(const CBlockHeader& block, BlockValida
                 *ppindex = pindex;
             if (pindex->nStatus & BLOCK_FAILED_MASK) {
                 LogDebug(BCLog::VALIDATION, "%s: block %s is marked invalid\n", __func__, hash.ToString());
-                return state.Invalid(BlockValidationResult::BLOCK_CACHED_INVALID, "duplicate");
+                return state.Invalid(BlockValidationResult::BLOCK_CACHED_INVALID, "duplicate-invalid");
             }
             return true;
         }