diff --git a/multibody/parsing/detail_tinyxml2_diagnostic.cc b/multibody/parsing/detail_tinyxml2_diagnostic.cc
index 708e1d4b9391..167bf64c919f 100644
--- a/multibody/parsing/detail_tinyxml2_diagnostic.cc
+++ b/multibody/parsing/detail_tinyxml2_diagnostic.cc
@@ -36,7 +36,10 @@ DiagnosticDetail TinyXml2Diagnostic::MakeDetail(
 
 void TinyXml2Diagnostic::Warning(
     const XMLNode& location, const std::string& message) const {
-  diagnostic_->Warning(MakeDetail(location, message));
+  // Avoid warnings with exactly the same message string.
+  if (warnings_.insert(message).second) {
+    diagnostic_->Warning(MakeDetail(location, message));
+  }
 }
 
 void TinyXml2Diagnostic::Error(
diff --git a/multibody/parsing/detail_tinyxml2_diagnostic.h b/multibody/parsing/detail_tinyxml2_diagnostic.h
index 37d4b40c883a..083088c2bcf0 100644
--- a/multibody/parsing/detail_tinyxml2_diagnostic.h
+++ b/multibody/parsing/detail_tinyxml2_diagnostic.h
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <string>
+#include <unordered_set>
 
 #include <tinyxml2.h>
 
@@ -56,6 +57,8 @@ class TinyXml2Diagnostic {
   const drake::internal::DiagnosticPolicy* diagnostic_{};
   const DataSource* data_source_{};
   const std::string file_extension_;
+  // Keep a history to avoid duplicate warnings.
+  mutable std::unordered_set<std::string> warnings_;
 };
 
 }  // namespace internal
diff --git a/multibody/parsing/test/detail_mujoco_parser_test.cc b/multibody/parsing/test/detail_mujoco_parser_test.cc
index f96f6d3485b2..952266f6d29e 100644
--- a/multibody/parsing/test/detail_mujoco_parser_test.cc
+++ b/multibody/parsing/test/detail_mujoco_parser_test.cc
@@ -1735,16 +1735,12 @@ TEST_F(MujocoParserTest, ContactWarnings) {
 
   AddModelFromString(xml, "test");
 
-  EXPECT_THAT(TakeWarning(), MatchesRegex(
-      ".*pair.*not have.*geom1.*geom2.*ignored.*"));
   EXPECT_THAT(TakeWarning(), MatchesRegex(
       ".*pair.*not have.*geom1.*geom2.*ignored.*"));
   EXPECT_THAT(TakeWarning(), MatchesRegex(".*pair.*unknown geom1.*ignored.*"));
   EXPECT_THAT(TakeWarning(), MatchesRegex(".*pair.*unknown geom2.*ignored.*"));
   EXPECT_THAT(TakeWarning(), MatchesRegex(
       ".*exclude.*not have.*body1.*body2.*ignored.*"));
-  EXPECT_THAT(TakeWarning(), MatchesRegex(
-      ".*exclude.*not have.*body1.*body2.*ignored.*"));
 }
 
 // TODO(rpoyner-tri) consider how to convert these to diagnostics.
diff --git a/multibody/parsing/test/detail_tinyxml2_diagnostic_test.cc b/multibody/parsing/test/detail_tinyxml2_diagnostic_test.cc
index e6ee913799fe..b66579d244d8 100644
--- a/multibody/parsing/test/detail_tinyxml2_diagnostic_test.cc
+++ b/multibody/parsing/test/detail_tinyxml2_diagnostic_test.cc
@@ -28,6 +28,7 @@ class TinyXml2DiagnosticTest : public test::DiagnosticPolicyTestBase {
   <warning unsupported_attribute='10'>
     <unsupported_element/>
   </warning>
+  <warning unsupported_attribute='20'/>
 </stuff>
 )""";
   }
@@ -121,19 +122,27 @@ TEST_F(TinyXml2DiagnosticContentsTest, PolicyForNode) {
 }
 
 TEST_F(TinyXml2DiagnosticContentsTest, Unsuppported) {
-  diagnostic_.WarnUnsupportedElement(
-      GetFirstChildNamed("warning"), "unsupported_element");
+  const XMLElement& warning_node = GetFirstChildNamed("warning");
+  diagnostic_.WarnUnsupportedElement(warning_node, "unsupported_element");
   EXPECT_EQ(TakeWarning(),
             "<literal-string>.stuff:6: warning: The tag 'unsupported_element'"
             " found as a child of 'warning' is currently unsupported and will"
             " be ignored.");
 
-  diagnostic_.WarnUnsupportedAttribute(
-      GetFirstChildNamed("warning"), "unsupported_attribute");
+  diagnostic_.WarnUnsupportedAttribute(warning_node, "unsupported_attribute");
   EXPECT_EQ(TakeWarning(),
             "<literal-string>.stuff:5: warning: The attribute"
             " 'unsupported_attribute' found in a 'warning' tag is currently"
             " unsupported and will be ignored.");
+
+  // Repeat the warning for the second instance of unsupported_element. This
+  // should *not* produce an additional warning, since it would produce a
+  // message that was identical except for the location in the file.
+  const XMLElement* repeated_warning_node =
+      warning_node.NextSiblingElement("warning");
+  ASSERT_NE(repeated_warning_node, nullptr);
+  diagnostic_.WarnUnsupportedAttribute(*repeated_warning_node,
+                                       "unsupported_attribute");
 }
 
 }  // namespace
diff --git a/multibody/parsing/test/detail_urdf_parser_test.cc b/multibody/parsing/test/detail_urdf_parser_test.cc
index 8059071cf7fd..9f62f8cbf16b 100644
--- a/multibody/parsing/test/detail_urdf_parser_test.cc
+++ b/multibody/parsing/test/detail_urdf_parser_test.cc
@@ -764,9 +764,7 @@ TEST_F(UrdfParserTest, TestAtlasMinimalContact) {
   const std::string full_name = FindRunfile(
       "drake_models/atlas/atlas_minimal_contact.urdf").abspath;
   AddModelFromUrdfFile(full_name, "");
-  for (int k = 0; k < 30; k++) {
-    EXPECT_THAT(TakeWarning(), MatchesRegex(".*safety_controller.*ignored.*"));
-  }
+  EXPECT_THAT(TakeWarning(), MatchesRegex(".*safety_controller.*ignored.*"));
   EXPECT_THAT(TakeWarning(), MatchesRegex(".*attached to a fixed joint.*"));
   plant_.Finalize();
 
@@ -798,7 +796,7 @@ TEST_F(UrdfParserTest, TestRegisteredSceneGraph) {
   // Test that registration with scene graph results in visual geometries.
   AddModelFromUrdfFile(full_name, "");
   // Mostly ignore warnings here; they are tested in detail elsewhere.
-  EXPECT_GT(warning_records_.size(), 30);
+  EXPECT_GT(warning_records_.size(), 1);
   warning_records_.clear();
   plant_.Finalize();
   EXPECT_NE(plant_.num_visual_geometries(), 0);