diff --git a/.gitignore b/.gitignore
index 8a9d35c..642e3ec 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 *.user
+.project_rules
diff --git a/README.md b/README.md
index 491e5f1..0d14855 100644
--- a/README.md
+++ b/README.md
@@ -336,7 +336,8 @@ This matters because the same numeric identifier must not be confused between st
 
 Immutable runtime snapshot with a private frame index. Build it with
 `DbcDecodeBuilder::Build(parsed)` or construct it from `std::vector<DecodeFrame>`.
-Construction validates all definitions before publishing the snapshot and throws
+Construction validates all definitions and duplicate keys before building the lookup
+index or publishing the snapshot, and throws
 `std::invalid_argument` for invalid metadata, including manually supplied data:
 
 - Normalized CAN IDs must fit 11 bits for standard frames or 29 bits for extended
diff --git a/changelog.md b/changelog.md
index 4a539f3..3efb19c 100644
--- a/changelog.md
+++ b/changelog.md
@@ -6,6 +6,20 @@ existing Git tag; subsequent changes are listed as unreleased.
 
 ## Unreleased
 
+### 2026-10-14
+
+#### Changed
+
+- Validate duplicate frame keys before building the runtime lookup index, alongside
+  CAN IDs, signal layouts, numeric parameters, and multiplexing metadata.
+
+#### Added
+
+- Exhaustive layout checks for both byte orders at representative payload sizes
+  (0, 1, 8, 64, and 255 bytes), including invalid lengths and uint32 overflow inputs.
+- Regression coverage for builder input mutation/destruction and failed replacement
+  with duplicate frame or signal keys, preserving existing lookups.
+
 ### 2026-10-13
 
 #### Added
diff --git a/core/decode_database.cpp b/core/decode_database.cpp
index bb8fc67..9956e43 100644
--- a/core/decode_database.cpp
+++ b/core/decode_database.cpp
@@ -89,12 +89,16 @@ namespace {
 DecodeDatabase::Storage::Storage(const std::vector<DecodeFrame> &definitions)
     : frames(definitions) {
     // Validate the complete snapshot before creating its lookup index.
-    for (const DecodeFrame &frame: frames)
+    std::unordered_set<FrameKey, FrameKeyHasher> keys;
+    keys.reserve(frames.size());
+    for (const DecodeFrame &frame: frames) {
         ValidateFrame(frame);
+        if (!keys.insert(frame.GetKey()).second)
+            throw std::invalid_argument("Duplicate runtime frame key");
+    }
     frameIndexByKey.reserve(frames.size());
     for (std::size_t index = 0U; index < frames.size(); ++index) {
-        if (!frameIndexByKey.emplace(frames[index].GetKey(), index).second)
-            throw std::invalid_argument("Duplicate runtime frame key");
+        frameIndexByKey.emplace(frames[index].GetKey(), index);
     }
 }
 
diff --git a/tests/README.md b/tests/README.md
index ffdefd8..225da77 100644
--- a/tests/README.md
+++ b/tests/README.md
@@ -160,3 +160,10 @@ lengths 1–64. Snapshot tests mutate retained frame, signal, and description al
 after rvalue construction, and verify unchanged lookups and metadata. Failed
 constructor/builder replacement preserves the active snapshot; clear removes its
 index while saved handles remain alive.
+
+Layout validation is also compared with independent DBC bit traversal for every
+start position through one byte beyond payload sizes 0, 1, 8, 64, and 255, and
+all lengths 1–64, for both byte orders. Zero/65-bit lengths and uint32 maximum
+inputs are rejected. Builder snapshots survive input mutation and destruction;
+failed replacement with duplicate frame or signal keys preserves the existing
+frame and signal lookups.
diff --git a/tests/dbc_validation_test.cpp b/tests/dbc_validation_test.cpp
index 6f1c574..5625424 100644
--- a/tests/dbc_validation_test.cpp
+++ b/tests/dbc_validation_test.cpp
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: MIT
 #include "dbc_decode_builder.h"
 #include "dbc_decoder.h"
+#include "signal_layout.h"
 #include <QtTest>
 #include <limits>
 #include <stdexcept>
@@ -53,6 +54,8 @@ private Q_SLOTS:
     void numericParameters_data();
     void numericParameters();
     void validBoundaries();
+    void layoutBoundaries();
+    void builderSnapshot();
     void multiplexingBoundaries();
     void independentSnapshot();
     void failedReplacement();
@@ -267,6 +270,50 @@ void DbcValidationTest::validBoundaries() {
     QVERIFY(DbcDecodeBuilder().Build(DbcDatabase()).GetFrames().empty());
 }
 
+void DbcValidationTest::layoutBoundaries() {
+    // Compare the shared coordinate calculation with independent DBC bit traversal.
+    for (const std::uint8_t dlc: {std::uint8_t(0U), std::uint8_t(1U), std::uint8_t(8U),
+                                  std::uint8_t(64U), std::uint8_t(255U)}) {
+        const std::uint64_t payloadBits = std::uint64_t(dlc) * 8U;
+        for (const bool littleEndian: {true, false}) {
+            for (std::uint32_t start = 0U; start < payloadBits + 8U; ++start) {
+                std::uint64_t bit = start;
+                bool fits = true;
+                QVERIFY(!IsValidSignalLayout(start, 0U, littleEndian, dlc));
+                for (std::uint32_t length = 1U; length <= 64U; ++length) {
+                    fits = fits && bit < payloadBits;
+                    QCOMPARE(IsValidSignalLayout(start, length, littleEndian, dlc), fits);
+                    if (littleEndian)
+                        ++bit;
+                    else
+                        bit = bit % 8U == 0U ? bit + 15U : bit - 1U;
+                }
+                QVERIFY(!IsValidSignalLayout(start, 65U, littleEndian, dlc));
+            }
+            const std::uint32_t maximum = std::numeric_limits<std::uint32_t>::max();
+            QVERIFY(!IsValidSignalLayout(maximum, 1U, littleEndian, dlc));
+            QVERIFY(!IsValidSignalLayout(maximum, maximum, littleEndian, dlc));
+            QVERIFY(!IsValidSignalLayout(0U, maximum, littleEndian, dlc));
+        }
+    }
+}
+
+void DbcValidationTest::builderSnapshot() {
+    DbcDatabase source = ParsedDefinitions({ValidFrame()});
+    source.frames[0].signals[0].valueDescriptions.emplace(std::uint64_t(1U), "One");
+    const FrameKey key(source.frames[0].canId, source.frames[0].isExtended);
+    const DecodeDatabase database = DbcDecodeBuilder().Build(source);
+    source.frames[0].canId = 0x800U;
+    source.frames[0].signals[0].name = "Changed";
+    source.frames[0].signals[0].valueDescriptions.begin()->second = "Changed";
+    source.frames.clear();
+    const std::shared_ptr<const DecodeSignal> signal = database.FindSignal(SignalKey(key, "Value"));
+    QVERIFY(signal);
+    QCOMPARE(signal->valueDescriptions.begin()->second, std::string("One"));
+    QVERIFY(database.FindFrame(key));
+    QVERIFY(!database.FindFrame(FrameKey(0x800U, false)));
+}
+
 void DbcValidationTest::multiplexingBoundaries() {
     DecodeFrame frame = ValidFrame();
     frame.dlc = 9U;
@@ -333,6 +380,14 @@ void DbcValidationTest::failedReplacement() {
     QVERIFY_THROWS_EXCEPTION(std::invalid_argument,
                              database = DbcDecodeBuilder().Build(ParsedDefinitions({invalid})));
     QCOMPARE(database.FindFrame(valid.GetKey()).get(), original.get());
+    QVERIFY_THROWS_EXCEPTION(std::invalid_argument, database = DecodeDatabase({valid, valid}));
+    QCOMPARE(database.FindFrame(valid.GetKey()).get(), original.get());
+    invalid = valid;
+    invalid.signals.push_back(invalid.signals[0]);
+    QVERIFY_THROWS_EXCEPTION(std::invalid_argument,
+                             database = DbcDecodeBuilder().Build(ParsedDefinitions({invalid})));
+    QCOMPARE(database.FindFrame(valid.GetKey()).get(), original.get());
+    QVERIFY(database.FindSignal(SignalKey(valid.GetKey(), "Value")));
     QCOMPARE(database.GetFrames().size(), std::size_t(1U));
     database = DecodeDatabase();
     QVERIFY(database.GetFrames().empty());