diff --git a/src/ifcopenshell-python/ifcopenshell/validate.py b/src/ifcopenshell-python/ifcopenshell/validate.py
index 22e747f50d..19f14447c0 100644
--- a/src/ifcopenshell-python/ifcopenshell/validate.py
+++ b/src/ifcopenshell-python/ifcopenshell/validate.py
@@ -350,10 +350,28 @@ def validate(f: Union[ifcopenshell.file, str], logger: Logger, express_rules=Fal
         log_internal_cpp_errors(filename, logger)
 
     schema = ifcopenshell.ifcopenshell_wrapper.schema_by_name(f.schema_identifier)
+    used_guids: dict[str, ifcopenshell.entity_instance] = dict()
+
     for inst in f:
         if hasattr(logger, "set_state"):
             logger.set_state("instance", inst)
 
+        if hasattr(inst, "GlobalId"):
+            guid = inst.GlobalId
+            if guid is not None and guid in used_guids:
+                rule = "Rule IfcRoot.UR1:\n    The attribute GlobalId should be unique"
+                previous_element = used_guids[guid]
+                logger.error(
+                    "On instance:\n    %s\n   %s\n%s\nViolated by:\n    %s\n    %s",
+                    inst,
+                    annotate_inst_attr_pos(inst, 0),
+                    rule,
+                    previous_element,
+                    annotate_inst_attr_pos(previous_element, 0),
+                )
+            else:
+                used_guids[guid] = inst
+
         entity, attrs = get_entity_attributes(schema, inst.is_a())
 
         if entity.is_abstract():
diff --git a/src/ifcopenshell-python/test/fixtures/validate/fail-duplicated-guids-ifc2x3.ifc b/src/ifcopenshell-python/test/fixtures/validate/fail-duplicated-guids-ifc2x3.ifc
new file mode 100644
index 0000000000..9a6eb58e05
--- /dev/null
+++ b/src/ifcopenshell-python/test/fixtures/validate/fail-duplicated-guids-ifc2x3.ifc
@@ -0,0 +1,20 @@
+ISO-10303-21;
+HEADER;
+FILE_DESCRIPTION(('ViewDefinition [CoordinationView]'),'2;1');
+FILE_NAME('','2024-04-02T17:16:50',(),(),'IfcOpenShell v0.7.0-e38eafd03','IfcOpenShell v0.7.0-e38eafd03','');
+FILE_SCHEMA(('IFC2X3'));
+ENDSEC;
+DATA;
+#1=IFCACTORROLE(.USERDEFINED.,'CONTRIBUTOR',$);
+#2=IFCTELECOMADDRESS(.USERDEFINED.,$,'WEBPAGE',$,$,$,$,'https://ifcopenshell.org');
+#3=IFCORGANIZATION('IfcOpenShell','IfcOpenShell','IfcOpenShell is an open source software library that helps users and software developers to work with IFC data.',(#1),(#2));
+#4=IFCAPPLICATION(#3,'v0.7.0-e38eafd03','IfcOpenShell','IfcOpenShell');
+#5=IFCPERSON('HSeldon','Seldon','Hari',$,$,$,$,$);
+#6=IFCORGANIZATION('APTR','Aperture Science',$,$,$);
+#7=IFCPERSONANDORGANIZATION(#5,#6,$);
+#8=IFCOWNERHISTORY(#7,#4,.READWRITE.,.ADDED.,1712060210,#7,#4,1712060210);
+#9=IFCBUILDINGELEMENTPROXY('3pdqyORIn8KBHZAhhtJ72T',#8,$,$,$,$,$,$,$);
+#10=IFCOWNERHISTORY(#7,#4,.READWRITE.,.ADDED.,1712060210,#7,#4,1712060210);
+#11=IFCBUILDINGELEMENTPROXY('3pdqyORIn8KBHZAhhtJ72T',#10,$,$,$,$,$,$,$);
+ENDSEC;
+END-ISO-10303-21;
diff --git a/src/ifcopenshell-python/test/fixtures/validate/fail-duplicated-guids-ifc4.ifc b/src/ifcopenshell-python/test/fixtures/validate/fail-duplicated-guids-ifc4.ifc
new file mode 100644
index 0000000000..eeca2cf84f
--- /dev/null
+++ b/src/ifcopenshell-python/test/fixtures/validate/fail-duplicated-guids-ifc4.ifc
@@ -0,0 +1,11 @@
+ISO-10303-21;
+HEADER;
+FILE_DESCRIPTION(('ViewDefinition [CoordinationView]'),'2;1');
+FILE_NAME('','2024-04-02T17:16:50',(),(),'IfcOpenShell v0.7.0-e38eafd03','IfcOpenShell v0.7.0-e38eafd03','');
+FILE_SCHEMA(('IFC4'));
+ENDSEC;
+DATA;
+#1=IFCBUILDINGELEMENTPROXY('3pdqyORIn8KBHZAhhtJ72T',$,$,$,$,$,$,$,$);
+#2=IFCBUILDINGELEMENTPROXY('3pdqyORIn8KBHZAhhtJ72T',$,$,$,$,$,$,$,$);
+ENDSEC;
+END-ISO-10303-21;
diff --git a/src/ifcopenshell-python/test/fixtures/validate/fail-duplicated-guids-ifc4x3.ifc b/src/ifcopenshell-python/test/fixtures/validate/fail-duplicated-guids-ifc4x3.ifc
new file mode 100644
index 0000000000..9bf812583a
--- /dev/null
+++ b/src/ifcopenshell-python/test/fixtures/validate/fail-duplicated-guids-ifc4x3.ifc
@@ -0,0 +1,11 @@
+ISO-10303-21;
+HEADER;
+FILE_DESCRIPTION(('ViewDefinition [CoordinationView]'),'2;1');
+FILE_NAME('','2024-04-02T17:16:50',(),(),'IfcOpenShell v0.7.0-e38eafd03','IfcOpenShell v0.7.0-e38eafd03','');
+FILE_SCHEMA(('IFC4X3'));
+ENDSEC;
+DATA;
+#1=IFCBUILDINGELEMENTPROXY('3pdqyORIn8KBHZAhhtJ72T',$,$,$,$,$,$,$,$);
+#2=IFCBUILDINGELEMENTPROXY('3pdqyORIn8KBHZAhhtJ72T',$,$,$,$,$,$,$,$);
+ENDSEC;
+END-ISO-10303-21;
diff --git a/src/ifcopenshell-python/test/fixtures/validate/generate_00.py b/src/ifcopenshell-python/test/fixtures/validate/generate_00.py
new file mode 100644
index 0000000000..e552a18970
--- /dev/null
+++ b/src/ifcopenshell-python/test/fixtures/validate/generate_00.py
@@ -0,0 +1,26 @@
+import ifcopenshell
+import ifcopenshell.api
+from pathlib import Path
+
+
+guid1 = "3pdqyORIn8KBHZAhhtJ72T"
+guid2 = "1sEzC8v31DshmvW5t5P631"
+
+for schema in ("ifc2x3", "ifc4", "ifc4x3"):
+    for status in ("fail", "pass"):
+        f = ifcopenshell.file(schema=schema)
+
+        # setup IfcOwnerHistory
+        if schema == "ifc2x3":
+            ifcopenshell.api.run("owner.add_application", f)
+            person = ifcopenshell.api.run("owner.add_person", f)
+            organization = ifcopenshell.api.run("owner.add_organisation", f)
+            ifcopenshell.api.run("owner.add_person_and_organisation", f, person=person, organisation=organization)
+
+        wall1 = ifcopenshell.api.run("root.create_entity", f)
+        wall2 = ifcopenshell.api.run("root.create_entity", f)
+        wall1.GlobalId = guid1
+        wall2.GlobalId = guid1 if status == "fail" else guid2
+
+        filename = f"{status}-{'not-' if status == 'pass' else ''}duplicated-guids-{schema}.ifc"
+        f.write(Path(__file__).parent / filename)
diff --git a/src/ifcopenshell-python/test/fixtures/validate/pass-not-duplicated-guids-ifc2x3.ifc b/src/ifcopenshell-python/test/fixtures/validate/pass-not-duplicated-guids-ifc2x3.ifc
new file mode 100644
index 0000000000..37d71d03e4
--- /dev/null
+++ b/src/ifcopenshell-python/test/fixtures/validate/pass-not-duplicated-guids-ifc2x3.ifc
@@ -0,0 +1,20 @@
+ISO-10303-21;
+HEADER;
+FILE_DESCRIPTION(('ViewDefinition [CoordinationView]'),'2;1');
+FILE_NAME('','2024-04-02T17:16:50',(),(),'IfcOpenShell v0.7.0-e38eafd03','IfcOpenShell v0.7.0-e38eafd03','');
+FILE_SCHEMA(('IFC2X3'));
+ENDSEC;
+DATA;
+#1=IFCACTORROLE(.USERDEFINED.,'CONTRIBUTOR',$);
+#2=IFCTELECOMADDRESS(.USERDEFINED.,$,'WEBPAGE',$,$,$,$,'https://ifcopenshell.org');
+#3=IFCORGANIZATION('IfcOpenShell','IfcOpenShell','IfcOpenShell is an open source software library that helps users and software developers to work with IFC data.',(#1),(#2));
+#4=IFCAPPLICATION(#3,'v0.7.0-e38eafd03','IfcOpenShell','IfcOpenShell');
+#5=IFCPERSON('HSeldon','Seldon','Hari',$,$,$,$,$);
+#6=IFCORGANIZATION('APTR','Aperture Science',$,$,$);
+#7=IFCPERSONANDORGANIZATION(#5,#6,$);
+#8=IFCOWNERHISTORY(#7,#4,.READWRITE.,.ADDED.,1712060210,#7,#4,1712060210);
+#9=IFCBUILDINGELEMENTPROXY('3pdqyORIn8KBHZAhhtJ72T',#8,$,$,$,$,$,$,$);
+#10=IFCOWNERHISTORY(#7,#4,.READWRITE.,.ADDED.,1712060210,#7,#4,1712060210);
+#11=IFCBUILDINGELEMENTPROXY('1sEzC8v31DshmvW5t5P631',#10,$,$,$,$,$,$,$);
+ENDSEC;
+END-ISO-10303-21;
diff --git a/src/ifcopenshell-python/test/fixtures/validate/pass-not-duplicated-guids-ifc4.ifc b/src/ifcopenshell-python/test/fixtures/validate/pass-not-duplicated-guids-ifc4.ifc
new file mode 100644
index 0000000000..e839daaba6
--- /dev/null
+++ b/src/ifcopenshell-python/test/fixtures/validate/pass-not-duplicated-guids-ifc4.ifc
@@ -0,0 +1,11 @@
+ISO-10303-21;
+HEADER;
+FILE_DESCRIPTION(('ViewDefinition [CoordinationView]'),'2;1');
+FILE_NAME('','2024-04-02T17:16:50',(),(),'IfcOpenShell v0.7.0-e38eafd03','IfcOpenShell v0.7.0-e38eafd03','');
+FILE_SCHEMA(('IFC4'));
+ENDSEC;
+DATA;
+#1=IFCBUILDINGELEMENTPROXY('3pdqyORIn8KBHZAhhtJ72T',$,$,$,$,$,$,$,$);
+#2=IFCBUILDINGELEMENTPROXY('1sEzC8v31DshmvW5t5P631',$,$,$,$,$,$,$,$);
+ENDSEC;
+END-ISO-10303-21;
diff --git a/src/ifcopenshell-python/test/fixtures/validate/pass-not-duplicated-guids-ifc4x3.ifc b/src/ifcopenshell-python/test/fixtures/validate/pass-not-duplicated-guids-ifc4x3.ifc
new file mode 100644
index 0000000000..d96dc23002
--- /dev/null
+++ b/src/ifcopenshell-python/test/fixtures/validate/pass-not-duplicated-guids-ifc4x3.ifc
@@ -0,0 +1,11 @@
+ISO-10303-21;
+HEADER;
+FILE_DESCRIPTION(('ViewDefinition [CoordinationView]'),'2;1');
+FILE_NAME('','2024-04-02T17:16:50',(),(),'IfcOpenShell v0.7.0-e38eafd03','IfcOpenShell v0.7.0-e38eafd03','');
+FILE_SCHEMA(('IFC4X3'));
+ENDSEC;
+DATA;
+#1=IFCBUILDINGELEMENTPROXY('3pdqyORIn8KBHZAhhtJ72T',$,$,$,$,$,$,$,$);
+#2=IFCBUILDINGELEMENTPROXY('1sEzC8v31DshmvW5t5P631',$,$,$,$,$,$,$,$);
+ENDSEC;
+END-ISO-10303-21;