diff --git a/core/commonMain/src/kotlinx/serialization/SerializationExceptions.kt b/core/commonMain/src/kotlinx/serialization/SerializationExceptions.kt
index 99f7d0a76a..4440836720 100644
--- a/core/commonMain/src/kotlinx/serialization/SerializationExceptions.kt
+++ b/core/commonMain/src/kotlinx/serialization/SerializationExceptions.kt
@@ -133,3 +133,10 @@ internal constructor(message: String?) : SerializationException(message) {
     // This constructor is used by the generated serializers
     constructor(index: Int) : this("An unknown field for index $index")
 }
+
+/**
+ * Thrown when a map deserializer encounters a repeated map key (and configuration disallows this.)
+ */
+@ExperimentalSerializationApi
+public class DuplicateMapKeyException(public val key: Any?) :
+    SerializationException("Duplicate keys not allowed in maps. Key appeared twice: $key")
diff --git a/core/commonMain/src/kotlinx/serialization/encoding/Decoding.kt b/core/commonMain/src/kotlinx/serialization/encoding/Decoding.kt
index dc4aa2ab9e..f701eed86e 100644
--- a/core/commonMain/src/kotlinx/serialization/encoding/Decoding.kt
+++ b/core/commonMain/src/kotlinx/serialization/encoding/Decoding.kt
@@ -275,7 +275,7 @@ internal inline fun <T : Any> Decoder.decodeIfNullable(deserializer: Deserializa
  * [CompositeDecoder] is a part of decoding process that is bound to a particular structured part of
  * the serialized form, described by the serial descriptor passed to [Decoder.beginStructure].
  *
- * Typically, for unordered data, [CompositeDecoder] is used by a serializer withing a [decodeElementIndex]-based
+ * Typically, for unordered data, [CompositeDecoder] is used by a serializer within a [decodeElementIndex]-based
  * loop that decodes all the required data one-by-one in any order and then terminates by calling [endStructure].
  * Please refer to [decodeElementIndex] for example of such loop.
  *
@@ -558,6 +558,17 @@ public interface CompositeDecoder {
         deserializer: DeserializationStrategy<T?>,
         previousValue: T? = null
     ): T?
+
+    /**
+     * Called after a key has been read.
+     *
+     * This could be a map or set key, or anything otherwise intended to be
+     * distinct within the collection under normal circumstances.
+     *
+     * Implementations might use this as a hook for throwing an exception when
+     * duplicate keys are encountered.
+     */
+    public fun visitKey(key: Any?) { }
 }
 
 /**
diff --git a/core/commonMain/src/kotlinx/serialization/internal/CollectionSerializers.kt b/core/commonMain/src/kotlinx/serialization/internal/CollectionSerializers.kt
index fcf4cfa74f..f624c64ec3 100644
--- a/core/commonMain/src/kotlinx/serialization/internal/CollectionSerializers.kt
+++ b/core/commonMain/src/kotlinx/serialization/internal/CollectionSerializers.kt
@@ -98,6 +98,7 @@ public sealed class MapLikeSerializer<Key, Value, Collection, Builder : MutableM
 
     final override fun readElement(decoder: CompositeDecoder, index: Int, builder: Builder, checkIndex: Boolean) {
         val key: Key = decoder.decodeSerializableElement(descriptor, index, keySerializer)
+        decoder.visitKey(key)
         val vIndex = if (checkIndex) {
             decoder.decodeElementIndex(descriptor).also {
                 require(it == index + 1) { "Value must follow key in a map, index for key: $index, returned index for value: $it" }
diff --git a/formats/cbor/commonMain/src/kotlinx/serialization/cbor/internal/Encoding.kt b/formats/cbor/commonMain/src/kotlinx/serialization/cbor/internal/Encoding.kt
index b77a18c599..8f9f2d2ac3 100644
--- a/formats/cbor/commonMain/src/kotlinx/serialization/cbor/internal/Encoding.kt
+++ b/formats/cbor/commonMain/src/kotlinx/serialization/cbor/internal/Encoding.kt
@@ -198,7 +198,17 @@ internal class CborEncoder(private val output: ByteArrayOutput) {
 }
 
 private class CborMapReader(cbor: Cbor, decoder: CborDecoder) : CborListReader(cbor, decoder) {
+    /** Keys that have been seen so far while reading this map. */
+    private val seenKeys = mutableSetOf<Any?>()
+
     override fun skipBeginToken() = setSize(decoder.startMap() * 2)
+
+    override fun visitKey(key: Any?) {
+        val added = seenKeys.add(key)
+        if (!added) {
+            throw DuplicateMapKeyException(key)
+        }
+    }
 }
 
 private open class CborListReader(cbor: Cbor, decoder: CborDecoder) : CborReader(cbor, decoder) {
diff --git a/formats/cbor/commonTest/src/kotlinx/serialization/cbor/CborStrictModeTest.kt b/formats/cbor/commonTest/src/kotlinx/serialization/cbor/CborStrictModeTest.kt
new file mode 100644
index 0000000000..43b8ec308b
--- /dev/null
+++ b/formats/cbor/commonTest/src/kotlinx/serialization/cbor/CborStrictModeTest.kt
@@ -0,0 +1,20 @@
+package kotlinx.serialization.cbor
+
+import kotlinx.serialization.assertFailsWithMessage
+import kotlinx.serialization.decodeFromByteArray
+import kotlinx.serialization.HexConverter
+import kotlinx.serialization.DuplicateMapKeyException
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class CborStrictModeTest {
+
+    /** Duplicate keys are rejected. */
+    @Test
+    fun testDuplicateKeys() {
+        val duplicateKeys = HexConverter.parseHexBinary("A2617805617806")
+        assertFailsWithMessage<DuplicateMapKeyException>("Duplicate keys not allowed in maps") {
+            Cbor.decodeFromByteArray<Map<String, Long>>(duplicateKeys)
+        }
+    }
+}