diff --git a/blueprintcompiler/ast_utils.py b/blueprintcompiler/ast_utils.py
index 7bfacfb..13f6eb1 100644
--- a/blueprintcompiler/ast_utils.py
+++ b/blueprintcompiler/ast_utils.py
@@ -175,7 +175,9 @@ class AstNode:
         for child in self.children:
             yield from child.get_semantic_tokens()
 
-    def validate_unique_in_parent(self, error, check=None):
+    def validate_unique_in_parent(
+        self, error: str, check: T.Optional[T.Callable[["AstNode"], bool]] = None
+    ):
         for child in self.parent.children:
             if child is self:
                 break
diff --git a/blueprintcompiler/language/gobject_signal.py b/blueprintcompiler/language/gobject_signal.py
index 6cc0c5f..f08168f 100644
--- a/blueprintcompiler/language/gobject_signal.py
+++ b/blueprintcompiler/language/gobject_signal.py
@@ -24,6 +24,23 @@ from .contexts import ScopeCtx
 from .common import *
 
 
+class SignalFlag(AstNode):
+    grammar = AnyOf(
+        UseExact("flag", "swapped"),
+        UseExact("flag", "after"),
+    )
+
+    @property
+    def flag(self) -> str:
+        return self.tokens["flag"]
+
+    @validate()
+    def unique(self):
+        self.validate_unique_in_parent(
+            f"Duplicate flag '{self.flag}'", lambda x: x.flag == self.flag
+        )
+
+
 class Signal(AstNode):
     grammar = Statement(
         UseIdent("name"),
@@ -39,12 +56,7 @@ class Signal(AstNode):
         Match("(").expected("argument list"),
         Optional(UseIdent("object")).expected("object identifier"),
         Match(")").expected(),
-        ZeroOrMore(
-            AnyOf(
-                [Keyword("swapped"), UseLiteral("swapped", True)],
-                [Keyword("after"), UseLiteral("after", True)],
-            )
-        ),
+        ZeroOrMore(SignalFlag),
     )
 
     @property
@@ -55,6 +67,13 @@ class Signal(AstNode):
     def detail_name(self) -> T.Optional[str]:
         return self.tokens["detail_name"]
 
+    @property
+    def full_name(self) -> str:
+        if self.detail_name is None:
+            return self.name
+        else:
+            return self.name + "::" + self.detail_name
+
     @property
     def handler(self) -> str:
         return self.tokens["handler"]
@@ -63,13 +82,17 @@ class Signal(AstNode):
     def object_id(self) -> T.Optional[str]:
         return self.tokens["object"]
 
+    @property
+    def flags(self) -> T.List[SignalFlag]:
+        return self.children[SignalFlag]
+
     @property
     def is_swapped(self) -> bool:
-        return self.tokens["swapped"] or False
+        return any(x.flag == "swapped" for x in self.flags)
 
     @property
     def is_after(self) -> bool:
-        return self.tokens["after"] or False
+        return any(x.flag == "after" for x in self.flags)
 
     @property
     def gir_signal(self):
diff --git a/blueprintcompiler/language/gtk_combo_box_text.py b/blueprintcompiler/language/gtk_combo_box_text.py
index 5725041..275327a 100644
--- a/blueprintcompiler/language/gtk_combo_box_text.py
+++ b/blueprintcompiler/language/gtk_combo_box_text.py
@@ -38,6 +38,13 @@ class Item(AstNode):
     def value(self) -> StringValue:
         return self.children[StringValue][0]
 
+    @validate("name")
+    def unique_in_parent(self):
+        if self.name is not None:
+            self.validate_unique_in_parent(
+                f"Duplicate item '{self.name}'", lambda x: x.name == self.name
+            )
+
 
 class ExtComboBoxItems(AstNode):
     grammar = [
diff --git a/blueprintcompiler/language/gtk_file_filter.py b/blueprintcompiler/language/gtk_file_filter.py
index daf0679..492f19e 100644
--- a/blueprintcompiler/language/gtk_file_filter.py
+++ b/blueprintcompiler/language/gtk_file_filter.py
@@ -46,6 +46,13 @@ class FilterString(AstNode):
     def item(self) -> str:
         return self.tokens["name"]
 
+    @validate()
+    def unique_in_parent(self):
+        self.validate_unique_in_parent(
+            f"Duplicate {self.tokens['tag_name']} '{self.item}'",
+            check=lambda child: child.item == self.item,
+        )
+
 
 def create_node(tag_name: str, singular: str):
     return Group(
diff --git a/blueprintcompiler/language/gtk_list_item_factory.py b/blueprintcompiler/language/gtk_list_item_factory.py
index cf1a415..bbb3bda 100644
--- a/blueprintcompiler/language/gtk_list_item_factory.py
+++ b/blueprintcompiler/language/gtk_list_item_factory.py
@@ -29,6 +29,10 @@ class ExtListItemFactory(AstNode):
             "sub-templates",
         )
 
+    @validate("template")
+    def unique_in_parent(self):
+        self.validate_unique_in_parent("Duplicate template block")
+
     @validate()
     def type_is_list_item(self):
         if self.type_name is not None:
diff --git a/blueprintcompiler/language/gtk_menu.py b/blueprintcompiler/language/gtk_menu.py
index b9de906..5698e12 100644
--- a/blueprintcompiler/language/gtk_menu.py
+++ b/blueprintcompiler/language/gtk_menu.py
@@ -69,6 +69,12 @@ class MenuAttribute(AstNode):
     def value_type(self) -> ValueTypeCtx:
         return ValueTypeCtx(None)
 
+    @validate("name")
+    def unique(self):
+        self.validate_unique_in_parent(
+            f"Duplicate attribute '{self.name}'", lambda x: x.name == self.name
+        )
+
 
 menu_child = AnyOf()
 
diff --git a/blueprintcompiler/language/gtk_size_group.py b/blueprintcompiler/language/gtk_size_group.py
index 235d92b..5ba4325 100644
--- a/blueprintcompiler/language/gtk_size_group.py
+++ b/blueprintcompiler/language/gtk_size_group.py
@@ -47,6 +47,12 @@ class Widget(AstNode):
                 f"Cannot assign {object.gir_class.full_name} to {type.full_name}"
             )
 
+    @validate("name")
+    def unique_in_parent(self):
+        self.validate_unique_in_parent(
+            f"Object '{self.name}' is listed twice", lambda x: x.name == self.name
+        )
+
 
 class ExtSizeGroupWidgets(AstNode):
     grammar = [
diff --git a/blueprintcompiler/language/gtk_styles.py b/blueprintcompiler/language/gtk_styles.py
index 798773c..6591aa5 100644
--- a/blueprintcompiler/language/gtk_styles.py
+++ b/blueprintcompiler/language/gtk_styles.py
@@ -29,6 +29,12 @@ class StyleClass(AstNode):
     def name(self) -> str:
         return self.tokens["name"]
 
+    @validate("name")
+    def unique_in_parent(self):
+        self.validate_unique_in_parent(
+            f"Duplicate style class '{self.name}'", lambda x: x.name == self.name
+        )
+
 
 class ExtStyles(AstNode):
     grammar = [
diff --git a/blueprintcompiler/language/gtkbuilder_child.py b/blueprintcompiler/language/gtkbuilder_child.py
index c3e1c47..4de13f2 100644
--- a/blueprintcompiler/language/gtkbuilder_child.py
+++ b/blueprintcompiler/language/gtkbuilder_child.py
@@ -112,6 +112,20 @@ class Child(AstNode):
         else:
             return None
 
+    @validate()
+    def internal_child_unique(self):
+        if self.annotation is not None:
+            if isinstance(self.annotation.child, ChildInternal):
+                internal_child = self.annotation.child.internal_child
+                self.validate_unique_in_parent(
+                    f"Duplicate internal child '{internal_child}'",
+                    lambda x: (
+                        x.annotation
+                        and isinstance(x.annotation.child, ChildInternal)
+                        and x.annotation.child.internal_child == internal_child
+                    ),
+                )
+
 
 @decompiler("child")
 def decompile_child(ctx, gir, type=None, internal_child=None):
diff --git a/blueprintcompiler/language/property_binding.py b/blueprintcompiler/language/property_binding.py
index 13bec61..686d1e8 100644
--- a/blueprintcompiler/language/property_binding.py
+++ b/blueprintcompiler/language/property_binding.py
@@ -44,6 +44,12 @@ class PropertyBindingFlag(AstNode):
                 actions=[CodeAction("remove 'sync-create'", "")],
             )
 
+    @validate()
+    def unique(self):
+        self.validate_unique_in_parent(
+            f"Duplicate flag '{self.flag}'", lambda x: x.flag == self.flag
+        )
+
 
 class PropertyBinding(AstNode):
     grammar = AnyOf(
diff --git a/blueprintcompiler/language/values.py b/blueprintcompiler/language/values.py
index cf88249..d0f3be5 100644
--- a/blueprintcompiler/language/values.py
+++ b/blueprintcompiler/language/values.py
@@ -229,6 +229,12 @@ class Flag(AstNode):
                 did_you_mean=(self.tokens["value"], expected_type.members.keys()),
             )
 
+    @validate()
+    def unique(self):
+        self.validate_unique_in_parent(
+            f"Duplicate flag '{self.name}'", lambda x: x.name == self.name
+        )
+
 
 class Flags(AstNode):
     grammar = [Flag, "|", Flag, ZeroOrMore(["|", Flag])]
diff --git a/tests/sample_errors/duplicate_internal_child.blp b/tests/sample_errors/duplicate_internal_child.blp
new file mode 100644
index 0000000..339ed0c
--- /dev/null
+++ b/tests/sample_errors/duplicate_internal_child.blp
@@ -0,0 +1,8 @@
+using Gtk 4.0;
+
+$MyWidget {
+  [internal-child test]
+  Button {}
+  [internal-child test]
+  Button {}
+}
\ No newline at end of file
diff --git a/tests/sample_errors/duplicate_internal_child.err b/tests/sample_errors/duplicate_internal_child.err
new file mode 100644
index 0000000..8bb16e7
--- /dev/null
+++ b/tests/sample_errors/duplicate_internal_child.err
@@ -0,0 +1 @@
+6,3,33,Duplicate internal child 'test'
\ No newline at end of file