Fix for 20303.

src/EFCore/Metadata/Internal/InternalForeignKeyBuilder.cs

@@ -1397,6 +1397,16 @@ public virtual InternalForeignKeyBuilder IsUnique(bool? unique, ConfigurationSou
                 return null;
             }
 
+            if (resetToDependent
+                && Metadata.PrincipalToDependent.GetConfigurationSource() == ConfigurationSource.Explicit)
+            {
+                throw new InvalidOperationException(
+                    CoreStrings.UnableToSetIsUnique(
+                        unique.Value,
+                        Metadata.PrincipalToDependent.PropertyInfo.Name,
+                        Metadata.PrincipalEntityType.DisplayName()));
+            }
+
             using var batch = Metadata.DeclaringEntityType.Model.ConventionDispatcher.DelayConventions();
             var builder = this;
             if (resetToDependent)

src/EFCore/Properties/CoreStrings.resx

@@ -1369,4 +1369,7 @@
   <data name="QueryEntityMaterializationConditionWrongShape" xml:space="preserve">
     <value>Materialization condition passed for entity shaper of entity type '{entityType}' is not of correct shape. Materialization condition must be LambdaExpression of 'Func&lt;ValueBuffer, IEntityType&gt;'</value>
   </data>
+  <data name="UnableToSetIsUnique" xml:space="preserve">
+    <value>Unable to set IsUnique to '{isUnique}' on the relationship underlying the navigation property '{navigationName}' on the entity type '{entityType}' because the navigation property has the opposite multiplicity.'</value>
+  </data>
 </root>

test/EFCore.Tests/ModelBuilding/OwnedTypesTestBase.cs

@@ -190,17 +190,21 @@ public virtual void Changing_ownership_uniqueness_throws()
             {
                 var modelBuilder = CreateModelBuilder();
 
-                modelBuilder.Entity<Customer>().OwnsOne(
-                    c => c.Details,
-                    r =>
-                    {
-                        r.HasOne(d => d.Customer)
-                            .WithMany();
-                    });
-
                 Assert.Equal(
-                    CoreStrings.NavigationNotAdded(nameof(Customer), nameof(Customer.Details), nameof(CustomerDetails)),
-                    Assert.Throws<InvalidOperationException>(() => modelBuilder.FinalizeModel()).Message);
+                    CoreStrings.UnableToSetIsUnique(
+                        false,
+                        nameof(Customer.Details),
+                        nameof(Customer)),
+                    Assert.Throws<InvalidOperationException>(
+                        () => modelBuilder
+                                .Entity<Customer>()
+                                .OwnsOne(
+                                    c => c.Details,
+                                    r =>
+                                    {
+                                        r.HasOne(d => d.Customer)
+                                            .WithMany();
+                                    })).Message);
             }
 
             [ConditionalFact]
@@ -1437,6 +1441,39 @@ public virtual void Navigations_on_OwnsMany_Owned_types_can_set_access_mode_usin
                 Assert.Equal(PropertyAccessMode.Field, principal.FindNavigation("OwnedDependents").GetPropertyAccessMode());
                 Assert.Equal(PropertyAccessMode.Property, dependent.FindNavigation("OneToManyOwner").GetPropertyAccessMode());
             }
+
+            [ConditionalFact]
+            public virtual void Attempt_to_create_OwnsMany_on_a_reference_throws()
+            {
+                var modelBuilder = CreateModelBuilder();
+
+                Assert.Equal(
+                    CoreStrings.UnableToSetIsUnique(
+                        false,
+                        "OwnedDependent",
+                        typeof(OneToOneNavPrincipalOwner).Name),
+                    Assert.Throws<InvalidOperationException>(
+                        () => modelBuilder
+                                .Entity<OneToOneNavPrincipalOwner>()
+                                .OwnsMany<OwnedNavDependent>("OwnedDependent")).Message
+                 );
+            }
+
+            [ConditionalFact]
+            public virtual void Attempt_to_create_OwnsOne_on_a_collection_throws()
+            {
+                var modelBuilder = CreateModelBuilder();
+
+                Assert.Equal(
+                    CoreStrings.UnableToSetIsUnique(
+                        true,
+                        "OwnedDependents",
+                        typeof(OneToManyNavPrincipalOwner).Name),
+                    Assert.Throws<InvalidOperationException>(
+                        () => modelBuilder
+                                .Entity<OneToManyNavPrincipalOwner>()
+                                .OwnsOne<OwnedOneToManyNavDependent>("OwnedDependents")).Message);
+            }
         }
     }
 }