diff --git a/src/compose/compose-doc.ts b/src/compose/compose-doc.ts
index cc65702e..e55c6cd3 100644
--- a/src/compose/compose-doc.ts
+++ b/src/compose/compose-doc.ts
@@ -24,6 +24,7 @@ export function composeDoc(
   const opts = Object.assign({ directives }, options)
   const doc = new Document(undefined, opts) as Document.Parsed
   const ctx: ComposeContext = {
+    anchors: options.version === 'next' ? new Map() : null,
     atRoot: true,
     directives: doc.directives,
     options: doc.options,
diff --git a/src/compose/compose-node.ts b/src/compose/compose-node.ts
index 6785777d..071f45a1 100644
--- a/src/compose/compose-node.ts
+++ b/src/compose/compose-node.ts
@@ -11,6 +11,7 @@ import { resolveEnd } from './resolve-end.js'
 import { emptyScalarPosition } from './util-empty-scalar-position.js'
 
 export interface ComposeContext {
+  anchors: Map<string, ParsedNode> | null
   atRoot: boolean
   directives: Directives
   options: Readonly<Required<Omit<ParseOptions, 'lineCounter'>>>
@@ -51,13 +52,13 @@ export function composeNode(
     case 'double-quoted-scalar':
     case 'block-scalar':
       node = composeScalar(ctx, token, tag, onError)
-      if (anchor) node.anchor = anchor.source.substring(1)
+      if (anchor) setAnchor(ctx, node, anchor, onError)
       break
     case 'block-map':
     case 'block-seq':
     case 'flow-collection':
       node = composeCollection(CN, ctx, token, tag, onError)
-      if (anchor) node.anchor = anchor.source.substring(1)
+      if (anchor) setAnchor(ctx, node, anchor, onError)
       break
     default: {
       const message =
@@ -134,3 +135,21 @@ function composeAlias(
   if (re.comment) alias.comment = re.comment
   return alias as Alias.Parsed
 }
+
+function setAnchor(
+  { anchors }: ComposeContext,
+  node: ParsedNode,
+  anchor: SourceToken,
+  onError: ComposeErrorHandler
+) {
+  const name = anchor.source.substring(1)
+  if (anchors) {
+    if (anchors.has(name)) {
+      const msg = `Anchors must be unique, ${name} is repeated`
+      onError(node.range, 'DUPLICATE_ANCHOR', msg)
+    } else {
+      anchors.set(name, node)
+    }
+  }
+  node.anchor = name
+}
diff --git a/src/errors.ts b/src/errors.ts
index a1db8a9c..84551e1b 100644
--- a/src/errors.ts
+++ b/src/errors.ts
@@ -10,6 +10,7 @@ export type ErrorCode =
   | 'BAD_SCALAR_START'
   | 'BLOCK_AS_IMPLICIT_KEY'
   | 'BLOCK_IN_FLOW'
+  | 'DUPLICATE_ANCHOR'
   | 'DUPLICATE_KEY'
   | 'IMPOSSIBLE'
   | 'KEY_OVER_1024_CHARS'
diff --git a/tests/next.ts b/tests/next.ts
index 1ee46d9c..f4e4bf7e 100644
--- a/tests/next.ts
+++ b/tests/next.ts
@@ -53,3 +53,24 @@ describe('relative-path alias', () => {
     expect(() => doc.toJS()).toThrow(ReferenceError)
   })
 })
+
+describe('unique anchors', () => {
+  test('repeats are fine without flag', () => {
+    const src = source`
+      - &a 1
+      - &a 2
+      - *a
+    `
+    expect(parse(src)).toEqual([1, 2, 2])
+  })
+
+  test("repeats are an error with 'next'", () => {
+    const src = source`
+      - &a 1
+      - &a 2
+      - *a
+    `
+    const doc = parseDocument(src, { version: 'next' })
+    expect(doc.errors).toMatchObject([{ code: 'DUPLICATE_ANCHOR' }])
+  })
+})