diff --git a/rodi/__init__.py b/rodi/__init__.py
index 21f4f0e..ffead96 100644
--- a/rodi/__init__.py
+++ b/rodi/__init__.py
@@ -475,7 +475,12 @@ def _get_resolver(self, desired_type, context: ResolutionContext):
         assert (
             reg is not None
         ), f"A resolver for type {class_name(desired_type)} is not configured"
-        return reg(context)
+        resolver = reg(context)
+
+        # add the resolver to the context, so we can find it
+        # next time we need it
+        context.resolved[desired_type] = resolver
+        return resolver
 
     def _get_resolvers_for_parameters(
         self,
@@ -567,11 +572,12 @@ def _ignore_class_attribute(self, key: str, value) -> bool:
         """
         Returns a value indicating whether a class attribute should be ignored for
         dependency resolution, by name and value.
+        It's ignored if it's a ClassVar or if it's already initialized explicitly.
         """
-        try:
-            return value.__origin__ is ClassVar
-        except AttributeError:
-            return False
+        is_classvar = getattr(value, "__origin__", None) is ClassVar
+        is_initialized = getattr(self.concrete_type, key, None) is not None
+
+        return is_classvar or is_initialized
 
     def _resolve_by_annotations(
         self, context: ResolutionContext, annotations: Dict[str, Type]
@@ -1146,14 +1152,24 @@ def build_provider(self) -> Services:
             _map: Dict[Union[str, Type], Type] = {}
 
             for _type, resolver in self._map.items():
-                # NB: do not call resolver if one was already prepared for the type
-                assert _type not in context.resolved, "_map keys must be unique"
-
                 if isinstance(resolver, DynamicResolver):
                     context.dynamic_chain.clear()
 
-                _map[_type] = resolver(context)
-                context.resolved[_type] = _map[_type]
+                if _type in context.resolved:
+                    # assert _type not in context.resolved, "_map keys must be unique"
+                    # check if its in the map
+                    if _type in _map:
+                        # NB: do not call resolver if one was already prepared for the
+                        # type
+                        raise OverridingServiceException(_type, resolver)
+                    else:
+                        resolved = context.resolved[_type]
+                else:
+                    # add to context so that we don't repeat operations
+                    resolved = resolver(context)
+                    context.resolved[_type] = resolved
+
+                _map[_type] = resolved
 
                 type_name = class_name(_type)
                 if "." not in type_name:
diff --git a/tests/test_examples.py b/tests/test_examples.py
index acaae27..0a7b3c6 100644
--- a/tests/test_examples.py
+++ b/tests/test_examples.py
@@ -12,6 +12,11 @@
 
 @pytest.mark.parametrize("file_path", examples)
 def test_example(file_path: str):
-    module_name = file_path.replace("./examples/", "").replace(".py", "")
+    module_name = (
+        # Windows
+        file_path.replace("./examples\\", "")
+        # Unix
+        .replace("./examples/", "").replace(".py", "")
+    )
     # assertions are in imported modules
     importlib.import_module(module_name)
diff --git a/tests/test_services.py b/tests/test_services.py
index f93fc69..9430813 100644
--- a/tests/test_services.py
+++ b/tests/test_services.py
@@ -2539,3 +2539,92 @@ class A:
     a = container.resolve(A)
 
     assert a.foo == "foo"
+
+
+def test_singleton_register_order_last():
+    """
+    The registration order of singletons should not matter.
+    Check that singletons are not registered twice when they are registered
+    after their dependents.
+    """
+
+    class Bar:
+        foo: Foo
+
+    class Bar2:
+        foo: Foo
+
+    container = Container()
+    container.register(Bar)
+    container.register(Bar2)
+    container._add_exact_singleton(Foo)
+
+    bar = container.resolve(Bar)
+    bar2 = container.resolve(Bar2)
+    foo = container.resolve(Foo)
+
+    # check that singletons are always the same instance
+    assert bar.foo is bar2.foo is foo
+
+
+def test_singleton_register_order_first():
+    """
+    The registration order of singletons should not matter.
+    Check that singletons are not registered twice when they are registered
+    before their dependents.
+    """
+
+    class Bar:
+        foo: Foo
+
+    class Bar2:
+        foo: Foo
+
+    container = Container()
+    container._add_exact_singleton(Foo)
+    container.register(Bar)
+    container.register(Bar2)
+
+    bar = container.resolve(Bar)
+    bar2 = container.resolve(Bar2)
+    foo = container.resolve(Foo)
+
+    # check that singletons are always the same instance
+    assert bar.foo is bar2.foo is foo
+
+
+def test_ignore_class_variable_if_already_initialized():
+    """
+    if a class variable is already initialized, it should not be overridden by
+    resolving a new instance nor fail if rodi can't resolve it.
+    """
+
+    foo_instance = Foo()
+
+    class A:
+        foo: Foo = foo_instance
+
+    class B:
+        example: ClassVar[str] = "example"
+        dependency: A
+
+    container = Container()
+
+    container.register(A)
+    container.register(B)
+    container._add_exact_singleton(Foo)
+
+    b = container.resolve(B)
+    a = container.resolve(A)
+    foo = container.resolve(Foo)
+
+    assert isinstance(a, A)
+    assert isinstance(a.foo, Foo)
+    assert foo_instance is a.foo
+
+    assert isinstance(b, B)
+    assert b.example == "example"
+    assert b.dependency.foo is foo_instance
+
+    # check that is not being overridden by resolving a new instance
+    assert foo is not a.foo