diff --git a/include/bitcoin/system/chain/block.hpp b/include/bitcoin/system/chain/block.hpp
index 15ad80ce0b..90a9956dcb 100644
--- a/include/bitcoin/system/chain/block.hpp
+++ b/include/bitcoin/system/chain/block.hpp
@@ -90,6 +90,7 @@ class BC_API block
     uint64_t fees() const NOEXCEPT;
     uint64_t claim() const NOEXCEPT;
     hash_digest hash() const NOEXCEPT;
+    bool is_malleable() const NOEXCEPT;
     bool is_segregated() const NOEXCEPT;
     size_t serialized_size(bool witness) const NOEXCEPT;
     size_t signature_operations(bool bip16, bool bip141) const NOEXCEPT;
diff --git a/include/bitcoin/system/error/block_error_t.hpp b/include/bitcoin/system/error/block_error_t.hpp
index 59dd0607f8..ea7c0fdd94 100644
--- a/include/bitcoin/system/error/block_error_t.hpp
+++ b/include/bitcoin/system/error/block_error_t.hpp
@@ -55,6 +55,7 @@ enum block_error_t : uint8_t
     forward_reference,
     merkle_mismatch,
     block_legacy_sigop_limit,
+    malleable_identifier,
 
     // accept block
     block_non_final,
diff --git a/src/chain/block.cpp b/src/chain/block.cpp
index a592359523..42c40878e1 100644
--- a/src/chain/block.cpp
+++ b/src/chain/block.cpp
@@ -274,7 +274,12 @@ size_t block::serialized_size(bool witness) const NOEXCEPT
 // Connect.
 // ----------------------------------------------------------------------------
 
-////// Subset of is_internal_double_spend if sha256 collisions cannot happen.
+// Subset of is_internal_double_spend if sha256 collisions cannot happen. This
+// is because each tx must have an input and for there to be no double spend in
+// the block the inputs must be unique (and only one coinbase). As the
+// is_internal_double_spend check invalidates any block with duplicated txs,
+// there can be no tx hash duplication within the merkle tree. And a block that
+// fails block.check is not archived, and its header remains potentially valid.
 ////bool block::is_distinct_transaction_set() const
 ////{
 ////    // A set is used to collapse duplicates.
@@ -450,6 +455,30 @@ bool block::is_hash_limit_exceeded() const NOEXCEPT
     return hashes.size() > hash_limit;
 }
 
+// This is not part of validation. Should be called after *invalidation* to
+// determine if the invalidity is universal (otherwise do not cache invalid).
+// lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-February/016697.html
+bool block::is_malleable() const NOEXCEPT
+{
+    // A two tx block cannot be malleable as coinbase is singular, otherwise
+    // if the last two non-witness tx hashes match then the id is malleable.
+    const auto count = txs_->size();
+    if (count > two && is_even(count) &&
+        txs_->at(sub1(count))->hash(false) == txs_->at(count)->hash(false))
+    {
+        return true;
+    }
+
+    // Hash of two same concatenated leaves is same as doubling one odd leaf.
+    const auto two_leaf_size = [](const transaction::cptr& tx) NOEXCEPT
+    {
+        return tx->serialized_size(false) == two * hash_size;
+    };
+
+    // If all non-witness tx serializations are 64 bytes the id is malleable.
+    return std::all_of(txs_->begin(), txs_->end(), two_leaf_size);
+}
+
 bool block::is_segregated() const NOEXCEPT
 {
     const auto segregated = [](const transaction::cptr& tx) NOEXCEPT
@@ -666,8 +695,8 @@ code block::check() const NOEXCEPT
 {
     // context free.
     // empty_block is redundant with first_not_coinbase.
-    ////if (is_empty())
-    ////    return error::empty_block;
+    //if (is_empty())
+    //    return error::empty_block;
     if (is_oversized())
         return error::block_size_limit;
     if (is_first_non_coinbase())
diff --git a/src/error/block_error_t.cpp b/src/error/block_error_t.cpp
index 456ee9e6e5..739cbd075a 100644
--- a/src/error/block_error_t.cpp
+++ b/src/error/block_error_t.cpp
@@ -49,6 +49,7 @@ DEFINE_ERROR_T_MESSAGE_MAP(block_error)
     { forward_reference, "transactions out of order" },
     { merkle_mismatch, "merkle root mismatch" },
     { block_legacy_sigop_limit, "too many block legacy signature operations" },
+    { malleable_identifier, "block identifier is malleable" },
 
     // accept block
     { block_non_final, "block contains a non-final transaction" },
diff --git a/test/chain/block.cpp b/test/chain/block.cpp
index 02e774eab5..de3d0e9de2 100644
--- a/test/chain/block.cpp
+++ b/test/chain/block.cpp
@@ -312,6 +312,7 @@ BOOST_AUTO_TEST_CASE(block__hash__default__matches_header_hash)
     BOOST_REQUIRE_EQUAL(instance.hash(), instance.header().hash());
 }
 
+// is_malleable
 // is_segregated
 // serialized_size
 
diff --git a/test/error/block_error_t.cpp b/test/error/block_error_t.cpp
index e6f9fffb8f..b55b4b54c1 100644
--- a/test/error/block_error_t.cpp
+++ b/test/error/block_error_t.cpp
@@ -172,6 +172,15 @@ BOOST_AUTO_TEST_CASE(block_error_t__code__block_legacy_sigop_limit__true_exected
     BOOST_REQUIRE_EQUAL(ec.message(), "too many block legacy signature operations");
 }
 
+BOOST_AUTO_TEST_CASE(block_error_t__code__malleable_identifier__true_exected_message)
+{
+    constexpr auto value = error::malleable_identifier;
+    const auto ec = code(value);
+    BOOST_REQUIRE(ec);
+    BOOST_REQUIRE(ec == value);
+    BOOST_REQUIRE_EQUAL(ec.message(), "block identifier is malleable");
+}
+
 // accept block
 
 BOOST_AUTO_TEST_CASE(block_error_t__code__block_non_final__true_exected_message)