diff --git a/CHANGES.rst b/CHANGES.rst
index f63b059e2..c88b161ca 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -43,6 +43,8 @@ Unreleased
     are closed to prevent a ``ResourceWarning``. :pr:`3101`
 -   ``SpooledTemporaryFile`` is always used for multipart file parsing.
     :pr:`3101`
+-   Raise a ``DuplicateRuleError`` when attempting to add a rule to a map with
+    an equal rule. :issue:`3037`
 
 
 Version 3.1.5
diff --git a/docs/routing.rst b/docs/routing.rst
index d295b3a0c..5df9e9741 100644
--- a/docs/routing.rst
+++ b/docs/routing.rst
@@ -74,6 +74,9 @@ format ``<converter(arguments):name>``. ``converter`` and ``arguments``
 ``default`` converter is used (``string`` by default). The available
 converters are discussed below.
 
+Rule path parts are separated by ``/`` and matched individually, unless a
+converter opts in to matching ``/`` as well.
+
 Rules that end with a slash are "branches", others are "leaves". If
 ``strict_slashes`` is enabled (the default), visiting a branch URL
 without a trailing slash will redirect to the URL with a slash appended.
@@ -86,6 +89,18 @@ merged. If you want to disable ``merge_slashes`` for a :class:`Rule` or
 :class:`Map`, you'll also need to configure your web server
 appropriately.
 
+Besides the path, matching can also use the subdomain of a known base domain if
+``subdomain_matching`` is enabled (the default), or the full host (if
+``host_matching`` is enabled). The subdomain or host parts can also contain
+variables. Unlike the path, where multiple parts are separated by ``/``, the
+domain is always matched as a single part.
+
+If a duplicate rule is added to a map, a :exc:`.DuplicateRuleError` will be
+raised. Rules are compared based on their path, subdomain or host, and websocket
+mode. Variable parts are not equal if they use different converters, although
+this heuristic may not be perfect depending on what the different converters can
+actually match.
+
 
 Rule Priority
 =============
@@ -148,10 +163,17 @@ Maps, Rules and Adapters
 .. autoclass:: Rule
    :members: empty
 
+.. currentmodule:: werkzeug.routing.exceptions
+
+.. autoclass:: DuplicateRuleError
+    :members:
+
 
 Rule Factories
 ==============
 
+.. currentmodule:: werkzeug.routing
+
 .. autoclass:: RuleFactory
    :members: get_rules
 
diff --git a/src/werkzeug/routing/exceptions.py b/src/werkzeug/routing/exceptions.py
index eeabd4ed1..2967a521c 100644
--- a/src/werkzeug/routing/exceptions.py
+++ b/src/werkzeug/routing/exceptions.py
@@ -17,6 +17,29 @@
     from .rules import Rule
 
 
+class DuplicateRuleError(Exception):
+    """Raised if a :class:`.Rule` is added to a :class:`.Map` that is equal to
+    an existing rule in the map.
+
+    :param existing: The existing rule that was duplicated.
+    :param duplicate: The new rule that duplicates the existing rule.
+
+    .. versionadded:: 3.2
+    """
+
+    def __init__(self, existing: Rule, duplicate: Rule) -> None:
+        super().__init__()
+
+        self.existing = existing
+        """The existing rule that was duplicated."""
+
+        self.duplicate = duplicate
+        """The new rule that duplicates the existing rule."""
+
+    def __str__(self) -> str:
+        return str(self.existing)
+
+
 class RoutingException(Exception):
     """Special exceptions that require the application to redirect, notifying
     about missing urls, etc.
diff --git a/src/werkzeug/routing/matcher.py b/src/werkzeug/routing/matcher.py
index 9174061bb..ee7651db0 100644
--- a/src/werkzeug/routing/matcher.py
+++ b/src/werkzeug/routing/matcher.py
@@ -6,6 +6,7 @@
 from dataclasses import field
 
 from .converters import ValidationError
+from .exceptions import DuplicateRuleError
 from .exceptions import NoMatch
 from .exceptions import RequestAliasRedirect
 from .exceptions import RequestPath
@@ -50,6 +51,11 @@ def add(self, rule: Rule) -> None:
                     new_state = State()
                     state.dynamic.append((part, new_state))
                     state = new_state
+
+        for existing in state.rules:
+            if rule == existing:
+                raise DuplicateRuleError(existing, rule)
+
         state.rules.append(rule)
 
     def update(self) -> None:
diff --git a/src/werkzeug/routing/rules.py b/src/werkzeug/routing/rules.py
index 7f5c566bb..18cfba1b1 100644
--- a/src/werkzeug/routing/rules.py
+++ b/src/werkzeug/routing/rules.py
@@ -907,12 +907,25 @@ def build_compare_key(self) -> tuple[int, int, int]:
         return (1 if self.alias else 0, -len(self.arguments), -len(self.defaults or ()))
 
     def __eq__(self, other: object) -> bool:
-        return isinstance(other, type(self)) and self._trace == other._trace
+        if not isinstance(other, type(self)):
+            return NotImplemented
+
+        return self._parts == other._parts and self.websocket == other.websocket
 
     __hash__ = None  # type: ignore
 
     def __str__(self) -> str:
-        return self.rule
+        out = self.rule
+
+        if self.map.subdomain_matching and self.subdomain:
+            out = f"{self.subdomain} {out}"
+        elif self.map.host_matching and self.host:
+            out = f"{self.host} {out}"
+
+        if self.websocket:
+            out = f"websocket {out}"
+
+        return f"{out} → {self.endpoint}"
 
     def __repr__(self) -> str:
         if self.map is None:
diff --git a/tests/test_routing.py b/tests/test_routing.py
index e7c266658..4f31037cb 100644
--- a/tests/test_routing.py
+++ b/tests/test_routing.py
@@ -13,6 +13,7 @@
 from werkzeug.exceptions import MethodNotAllowed
 from werkzeug.exceptions import NotFound
 from werkzeug.exceptions import SecurityError
+from werkzeug.routing.exceptions import DuplicateRuleError
 from werkzeug.test import create_environ
 from werkzeug.wrappers import Request
 from werkzeug.wrappers import Response
@@ -251,6 +252,74 @@ def test_strict_slashes_leaves_dont_consume():
     assert adapter.match("/path5/", method="GET") == ("leaf", {})
 
 
+def test_no_duplicates() -> None:
+    """A rule's subdomain part and websocket mode are considered for equality."""
+    r.Map(
+        [
+            r.Rule("/", endpoint="index"),
+            r.Rule("/", endpoint="websocket", websocket=True),
+            r.Rule("/", endpoint="sub", subdomain="a"),
+        ]
+    )
+
+
+def test_no_duplicates_host() -> None:
+    """If host_matching is enabled, a rule's host is considered for equality."""
+    r.Map(
+        [
+            r.Rule("/", endpoint="index", host="a"),
+            r.Rule("/", endpoint="host", host="b"),
+        ],
+        host_matching=True,
+    )
+
+
+def test_duplicate_domain_disabled() -> None:
+    """If domain matching is disabled, a rule's subdomain is not considered for
+    equality.
+    """
+    with pytest.raises(DuplicateRuleError):
+        r.Map(
+            [
+                r.Rule("/", endpoint="index"),
+                r.Rule("/", endpoint="sub", subdomain="a"),
+            ],
+            subdomain_matching=False,
+        )
+
+
+def test_duplicate_rule() -> None:
+    """Equal static parts are duplicates."""
+    with pytest.raises(DuplicateRuleError):
+        r.Map(
+            [
+                r.Rule("/", endpoint="a"),
+                r.Rule("/", endpoint="b"),
+            ]
+        )
+
+
+def test_duplicate_converter() -> None:
+    """Equal converter parts are duplicates."""
+    with pytest.raises(DuplicateRuleError):
+        r.Map(
+            [
+                r.Rule("/page/<a>", endpoint="a"),
+                r.Rule("/page/<b>", endpoint="b"),
+            ]
+        )
+
+
+def test_no_duplicate_different_converters() -> None:
+    """Converters of different types are not duplicates."""
+    r.Map(
+        [
+            r.Rule("/<a>", endpoint="a"),
+            r.Rule("/<int:b>", endpoint="b"),
+        ]
+    )
+
+
 def test_environ_defaults():
     environ = create_environ("/foo")
     assert environ["PATH_INFO"] == "/foo"