diff --git a/SAX2.c b/SAX2.c
index 8e6ba6c7c..17c8e0e2e 100644
--- a/SAX2.c
+++ b/SAX2.c
@@ -1190,8 +1190,19 @@ xmlSAX1Attribute(xmlParserCtxtPtr ctxt, const xmlChar *fullname,
                 xmlFree(val);
 	    }
 	} else {
+            /*
+             * When replacing entities, make sure that IDs in
+             * entities aren't registered. This also shouldn't be
+             * done when entities aren't replaced, but this would
+             * require to rework IDREF checks.
+             */
+            if (ctxt->input->entity != NULL)
+                ctxt->vctxt.flags |= XML_VCTXT_IN_ENTITY;
+
 	    ctxt->valid &= xmlValidateOneAttribute(&ctxt->vctxt, ctxt->myDoc,
 					       ctxt->node, ret, value);
+
+            ctxt->vctxt.flags &= ~XML_VCTXT_IN_ENTITY;
 	}
     } else
 #endif /* LIBXML_VALID_ENABLED */
@@ -2057,8 +2068,19 @@ xmlSAX2AttributeNs(xmlParserCtxtPtr ctxt,
             if (dup == NULL)
                 xmlSAX2ErrMemory(ctxt);
 
+            /*
+             * When replacing entities, make sure that IDs in
+             * entities aren't registered. This also shouldn't be
+             * done when entities aren't replaced, but this would
+             * require to rework IDREF checks.
+             */
+            if (ctxt->input->entity != NULL)
+                ctxt->vctxt.flags |= XML_VCTXT_IN_ENTITY;
+
 	    ctxt->valid &= xmlValidateOneAttribute(&ctxt->vctxt,
 	                             ctxt->myDoc, ctxt->node, ret, dup);
+
+            ctxt->vctxt.flags &= ~XML_VCTXT_IN_ENTITY;
 	}
     } else
 #endif /* LIBXML_VALID_ENABLED */
diff --git a/include/private/parser.h b/include/private/parser.h
index a1c3d04a5..7066118ad 100644
--- a/include/private/parser.h
+++ b/include/private/parser.h
@@ -22,6 +22,10 @@
  * Set if the validation is enabled.
  */
 #define XML_VCTXT_VALIDATE (1u << 2)
+/**
+ * Set when parsing entities.
+ */
+#define XML_VCTXT_IN_ENTITY (1u << 3)
 
 /*
  * TODO: Rename to avoid confusion with xmlParserInputFlags
diff --git a/valid.c b/valid.c
index 30a29d715..f7c3b33e3 100644
--- a/valid.c
+++ b/valid.c
@@ -3962,7 +3962,8 @@ xmlValidateOneAttribute(xmlValidCtxt *ctxt, xmlDoc *doc,
     }
 
     /* Validity Constraint: ID uniqueness */
-    if (attrDecl->atype == XML_ATTRIBUTE_ID) {
+    if (attrDecl->atype == XML_ATTRIBUTE_ID &&
+        (ctxt->flags & XML_VCTXT_IN_ENTITY) == 0) {
         if (xmlAddID(ctxt, doc, value, attr) == NULL)
 	    ret = 0;
     }