Show only the duplicate keys in the exception message, not all keys.

Arguably we should also:
- Show the entire map.
- Fail the assertion instead of throwing.

But this CL seems like a strict improvement.

RELNOTES=n/a
PiperOrigin-RevId: 799515840

Author: cpovirk

core/src/main/java/com/google/common/truth/MapSubject.java

@@ -17,6 +17,7 @@
 
 import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Preconditions.checkNotNull;
+import static com.google.common.base.Strings.lenientFormat;
 import static com.google.common.collect.Maps.immutableEntry;
 import static com.google.common.truth.Fact.fact;
 import static com.google.common.truth.Fact.simpleFact;
@@ -28,6 +29,7 @@
 import static com.google.common.truth.SubjectUtils.retainMatchingToString;
 import static java.util.Collections.singletonList;
 
+import com.google.common.collect.FluentIterable;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.LinkedHashMultiset;
 import com.google.common.collect.Maps;
@@ -260,11 +262,14 @@ public final Ordered containsAtLeast(
       expectedMap.put(key, rest[i + 1]);
       keys.add(key);
     }
-    checkArgument(
-        keys.size() == expectedMap.size(),
-        "Duplicate keys (%s) cannot be passed to %s().",
-        keys,
-        functionName);
+    if (expectedMap.size() != keys.size()) {
+      List<Multiset.Entry<@Nullable Object>> duplicateKeys =
+          FluentIterable.from(keys.entrySet()).filter(e -> e.getCount() > 1).toList();
+      throw new IllegalArgumentException(
+          lenientFormat(
+              // TODO(cpovirk): Consider a format like SubjectUtils.entryString instead.
+              "Duplicate keys (%s) cannot be passed to %s().", duplicateKeys, functionName));
+    }
     return expectedMap;
   }
 

core/src/test/java/com/google/common/truth/MapSubjectTest.java

@@ -160,6 +160,19 @@ public void containsExactlyMultipleDuplicateKeys() {
         .isEqualTo("Duplicate keys ([jan x 2, feb x 2]) cannot be passed to containsExactly().");
   }
 
+  @Test
+  public void containsExactlyBothDuplicateAndNonDuplicateKeys() {
+    ImmutableMap<String, Integer> actual = ImmutableMap.of("jan", 1, "feb", 2);
+
+    IllegalArgumentException expected =
+        assertThrows(
+            IllegalArgumentException.class,
+            () -> assertThat(actual).containsExactly("jan", 1, "jan", 1, "feb", 2));
+    assertThat(expected)
+        .hasMessageThat()
+        .isEqualTo("Duplicate keys ([jan x 2]) cannot be passed to containsExactly().");
+  }
+
   @Test
   public void containsExactlyExtraKey() {
     ImmutableMap<String, Integer> actual = ImmutableMap.of("jan", 1, "feb", 2, "march", 3);