diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 7ca8770bd02..982fca8983d 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -15438,6 +15438,16 @@ lookup_and_check_tag (enum tag_types tag_code, tree name,
       return error_mark_node;
     }
 
+  if (DECL_CLASS_TEMPLATE_P (decl)
+      && !template_header_p
+      && how == TAG_how::CURRENT_ONLY)
+    {
+      error ("class template %qD redeclared as non-template", name);
+      inform (location_of (decl), "previous declaration here");
+      CLASSTYPE_ERRONEOUS (TREE_TYPE (decl)) = true;
+      return error_mark_node;
+    }
+
   if (DECL_CLASS_TEMPLATE_P (decl)
       /* If scope is TAG_how::CURRENT_ONLY we're defining a class,
 	 so ignore a template template parameter.  */
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 18c6f118ae6..4f0ae6d5851 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -29601,6 +29601,13 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
   if (DECL_TEMPLATE_TEMPLATE_PARM_P (tmpl))
     return ptype;
 
+  /* If the class was erroneous, don't try to deduce, because that
+     can generate a lot of diagnostic.  */
+  if (TREE_TYPE (tmpl)
+      && TYPE_LANG_SPECIFIC (TREE_TYPE (tmpl))
+      && CLASSTYPE_ERRONEOUS (TREE_TYPE (tmpl)))
+    return ptype;
+
   /* Wait until the enclosing scope is non-dependent.  */
   if (DECL_CLASS_SCOPE_P (tmpl)
       && dependent_type_p (DECL_CONTEXT (tmpl)))
diff --git a/gcc/testsuite/g++.dg/diagnostic/redeclaration-2.C b/gcc/testsuite/g++.dg/diagnostic/redeclaration-2.C
new file mode 100644
index 00000000000..4423e12b0b1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/diagnostic/redeclaration-2.C
@@ -0,0 +1,19 @@
+// PR c++/103749
+
+struct foo {
+  template<typename>
+  friend struct bar;
+};
+
+struct bar { // { dg-error "redeclared as non-template" }
+  int baz;
+};
+
+template<typename>
+struct T; // { dg-message "previous" }
+
+struct T { // { dg-error "redeclared as non-template" }
+};
+
+bar var; // { dg-error "" }
+T t; // { dg-error "" }
diff --git a/gcc/testsuite/g++.dg/template/redecl4.C b/gcc/testsuite/g++.dg/template/redecl4.C
index 5638bde413d..c9282cde860 100644
--- a/gcc/testsuite/g++.dg/template/redecl4.C
+++ b/gcc/testsuite/g++.dg/template/redecl4.C
@@ -2,4 +2,4 @@
 // { dg-do compile }
 
 template<int> union A;  // { dg-message "previous" }
-struct A;               // { dg-error "non-template" }
+struct A;               // { dg-error "redeclared as non-template" }