diff --git a/compiler/src/dotty/tools/dotc/core/tasty/AttributePickler.scala b/compiler/src/dotty/tools/dotc/core/tasty/AttributePickler.scala
index 5aea55bce4de..cd2756f4c752 100644
--- a/compiler/src/dotty/tools/dotc/core/tasty/AttributePickler.scala
+++ b/compiler/src/dotty/tools/dotc/core/tasty/AttributePickler.scala
@@ -14,13 +14,21 @@ object AttributePickler:
   ): Unit =
     pickler.newSection(AttributesSection, buf)
 
+    var lastTag = -1
+    def assertTagOrder(tag: Int): Unit =
+      assert(tag != lastTag, s"duplicate attribute tag: $tag")
+      assert(tag > lastTag, s"attribute tags are not ordered: $tag after $lastTag")
+      lastTag = tag
+
     for tag <- attributes.booleanTags do
       assert(isBooleanAttrTag(tag), "Not a boolean attribute tag: " + tag)
+      assertTagOrder(tag)
       buf.writeByte(tag)
 
     assert(attributes.stringTagValues.exists(_._1 == SOURCEFILEattr))
     for (tag, value) <- attributes.stringTagValues do
       assert(isStringAttrTag(tag), "Not a string attribute tag: " + tag)
+      assertTagOrder(tag)
       val utf8Ref = pickler.nameBuffer.utf8Index(value)
       buf.writeByte(tag)
       buf.writeNat(utf8Ref.index)
diff --git a/compiler/src/dotty/tools/dotc/core/tasty/AttributeUnpickler.scala b/compiler/src/dotty/tools/dotc/core/tasty/AttributeUnpickler.scala
index 405f110dbca1..43a2bea27216 100644
--- a/compiler/src/dotty/tools/dotc/core/tasty/AttributeUnpickler.scala
+++ b/compiler/src/dotty/tools/dotc/core/tasty/AttributeUnpickler.scala
@@ -15,6 +15,7 @@ class AttributeUnpickler(reader: TastyReader, nameAtRef: NameTable):
     val booleanTags = BitSet.newBuilder
     val stringTagValue = List.newBuilder[(Int, String)]
 
+    var lastTag = -1
     while !isAtEnd do
       val tag = readByte()
       if isBooleanAttrTag(tag) then
@@ -26,6 +27,11 @@ class AttributeUnpickler(reader: TastyReader, nameAtRef: NameTable):
       else
         assert(false, "unknown attribute tag: " + tag)
 
+      assert(tag != lastTag, s"duplicate attribute tag: $tag")
+      assert(tag > lastTag, s"attribute tags are not ordered: $tag after $lastTag")
+      lastTag = tag
+    end while
+
     new Attributes(booleanTags.result(), stringTagValue.result())
   }
 
diff --git a/tasty/src/dotty/tools/tasty/TastyFormat.scala b/tasty/src/dotty/tools/tasty/TastyFormat.scala
index e1a8cf1d50ae..adadd3267aac 100644
--- a/tasty/src/dotty/tools/tasty/TastyFormat.scala
+++ b/tasty/src/dotty/tools/tasty/TastyFormat.scala
@@ -281,6 +281,7 @@ Standard Section: "Attributes" Attribute*
                   OUTLINEattr
                   SOURCEFILEattr Utf8Ref
 ```
+Attribute tags cannot be repeated in an attribute section. Attributes are ordered by the tag ordinal.
 
 Note: Attribute tags are grouped into categories that determine what follows, and thus allow to compute the size of the tagged tree in a generic way.
       Unassigned categories can be used to extend and existing category or to add new kinds of attributes