diff --git a/src/shader/ShaderPackageRegistry.cpp b/src/shader/ShaderPackageRegistry.cpp
index 2517755..2a91ff2 100644
--- a/src/shader/ShaderPackageRegistry.cpp
+++ b/src/shader/ShaderPackageRegistry.cpp
@@ -77,6 +77,7 @@ bool ShaderPackageRegistry::Scan(
 		return false;
 	}
 
+	std::map<std::string, std::string> customUiTagOwners;
 	for (const auto& entry : std::filesystem::directory_iterator(shaderRoot))
 	{
 		if (!entry.is_directory())
@@ -100,6 +101,21 @@ bool ShaderPackageRegistry::Scan(
 			continue;
 		}
 
+		if (shaderPackage.ui.enabled)
+		{
+			const auto tagOwnerIt = customUiTagOwners.find(shaderPackage.ui.customElementTag);
+			if (tagOwnerIt != customUiTagOwners.end())
+			{
+				packageStatuses.push_back(BuildUnavailableStatus(
+					manifestPath,
+					shaderPackage,
+					"Duplicate shader UI custom element tag '" + shaderPackage.ui.customElementTag +
+						"' already used by shader '" + tagOwnerIt->second + "'."));
+				continue;
+			}
+			customUiTagOwners[shaderPackage.ui.customElementTag] = shaderPackage.id;
+		}
+
 		packageOrder.push_back(shaderPackage.id);
 		packageStatuses.push_back(BuildAvailableStatus(shaderPackage));
 		packagesById[shaderPackage.id] = shaderPackage;
diff --git a/tests/ShaderPackageRegistryTests.cpp b/tests/ShaderPackageRegistryTests.cpp
index b90210b..dcba966 100644
--- a/tests/ShaderPackageRegistryTests.cpp
+++ b/tests/ShaderPackageRegistryTests.cpp
@@ -138,6 +138,38 @@ void TestInvalidUiDefinition()
 	std::filesystem::remove_all(root);
 }
 
+void TestDuplicateCustomUiTags()
+{
+	const std::filesystem::path root = MakeTestRoot();
+	WriteFile(root / "a" / "ui" / "controls.js", "customElements.define('shared-controls', class extends HTMLElement {});\n");
+	WriteFile(root / "b" / "ui" / "controls.js", "customElements.define('shared-controls', class extends HTMLElement {});\n");
+	WriteShaderPackage(root, "a", R"({
+		"id": "shader-a",
+		"name": "Shader A",
+		"ui": { "type": "webComponent", "entry": "ui/controls.js", "tag": "shared-controls" },
+		"parameters": []
+	})");
+	WriteShaderPackage(root, "b", R"({
+		"id": "shader-b",
+		"name": "Shader B",
+		"ui": { "type": "webComponent", "entry": "ui/controls.js", "tag": "shared-controls" },
+		"parameters": []
+	})");
+
+	ShaderPackageRegistry registry(4);
+	std::map<std::string, ShaderPackage> packages;
+	std::vector<std::string> order;
+	std::vector<ShaderPackageStatus> statuses;
+	std::string error;
+	Expect(registry.Scan(root, packages, order, statuses, error), "duplicate custom UI tags do not fail the whole scan");
+	Expect(packages.size() == 1, "only one duplicate custom UI tag owner remains available");
+	Expect(std::any_of(statuses.begin(), statuses.end(), [](const ShaderPackageStatus& status) {
+		return !status.available && status.error.find("Duplicate shader UI custom element tag") != std::string::npos;
+	}), "duplicate custom UI tag is surfaced as an unavailable shader");
+
+	std::filesystem::remove_all(root);
+}
+
 void TestExplicitPassManifest()
 {
 	const std::filesystem::path root = MakeTestRoot();
@@ -335,6 +367,7 @@ int main()
 	TestValidManifest();
 	TestExplicitPassManifest();
 	TestInvalidUiDefinition();
+	TestDuplicateCustomUiTags();
 	TestMissingFontAsset();
 	TestInvalidManifest();
 	TestInvalidTemporalSettings();
diff --git a/ui/src/components/ShaderCustomPanel.jsx b/ui/src/components/ShaderCustomPanel.jsx
index 6218ce7..20d3394 100644
--- a/ui/src/components/ShaderCustomPanel.jsx
+++ b/ui/src/components/ShaderCustomPanel.jsx
@@ -140,7 +140,11 @@ export function ShaderCustomPanel({ layer, ui, onParameterChange, onResetParamet
             return;
           }
           if (element.parentNode !== node) {
-            node.replaceChildren(element);
+            try {
+              node.replaceChildren(element);
+            } catch (error) {
+              setLoadError(error instanceof Error ? error.message : "Custom controls failed to mount.");
+            }
           }
         }}
       >