diff --git a/src/__tests__/lib/thirdParty.test.ts b/src/__tests__/lib/thirdParty.test.ts
index 1c2e054f..c432fe75 100644
--- a/src/__tests__/lib/thirdParty.test.ts
+++ b/src/__tests__/lib/thirdParty.test.ts
@@ -134,6 +134,36 @@ describe("thirdParty script registry", () => {
     }
   });
 
+  it("handles script load errors via onError callback", () => {
+    const onError = jest.fn();
+    const mod = loadWithEnv({
+      NEXT_PUBLIC_ANALYTICS_PROVIDER: "plausible",
+      NEXT_PUBLIC_ANALYTICS_DOMAIN: "proofofheart.xyz",
+    });
+
+    // Script configs returned by the registry have no onError by default.
+    for (const s of mod.getThirdPartyScripts()) {
+      mod.handleScriptError(s);
+    }
+    expect(onError).not.toHaveBeenCalled();
+
+    // When a consumer wires onError, handleScriptError invokes it.
+    const script = mod.getThirdPartyScripts()[0];
+    mod.handleScriptError({ ...script, onError });
+    expect(onError).toHaveBeenCalledTimes(1);
+  });
+
+  it("does not return duplicate script ids", () => {
+    const mod = loadWithEnv({
+      NEXT_PUBLIC_ANALYTICS_PROVIDER: "plausible",
+      NEXT_PUBLIC_ANALYTICS_DOMAIN: "proofofheart.xyz",
+      NEXT_PUBLIC_SUPPORT_WIDGET_SRC: "https://widget.example.com/w.js",
+    });
+
+    const ids = mod.getThirdPartyScripts().map((s) => s.id);
+    expect(new Set(ids).size).toBe(ids.length);
+  });
+
   it("derives CSP origins from the configured scripts", () => {
     const mod = loadWithEnv({
       NEXT_PUBLIC_ANALYTICS_PROVIDER: "plausible",
diff --git a/src/components/ThirdPartyScripts.tsx b/src/components/ThirdPartyScripts.tsx
index ec5947b0..2b212fd3 100644
--- a/src/components/ThirdPartyScripts.tsx
+++ b/src/components/ThirdPartyScripts.tsx
@@ -38,34 +38,19 @@ export default function ThirdPartyScripts() {
 
   return (
     <>
-      {scripts.map(({ id, src, strategy, attributes }) => {
-        // #647 — Guard: `beforeInteractive` runs during SSR and blocks the main
-        // thread before hydration — exactly the problem this component exists to
-        // solve. Any misconfigured entry is demoted to `lazyOnload` at runtime
-        // and flagged in the dev console so it can be fixed in thirdParty.ts.
-        const safeStrategy =
-          strategy === "beforeInteractive"
-            ? (process.env.NODE_ENV !== "production" &&
-                console.warn(
-                  `[ThirdPartyScripts] Script "${id}" uses "beforeInteractive" which blocks` +
-                    ` the main thread. Downgraded to "lazyOnload". Fix the strategy in thirdParty.ts.`
-                ),
-              "lazyOnload" as const)
-            : strategy;
-
-        return (
-          <Script
-            key={id}
-            id={id}
-            src={src}
-            strategy={safeStrategy}
-            // Funnel events fired during hydration are buffered by `analytics.ts`
-            // until the vendor global exists; this is where they get drained.
-            onLoad={id === analyticsId ? flushAnalyticsQueue : undefined}
-            {...attributes}
-          />
-        );
-      })}
+      {scripts.map(({ id, src, strategy, attributes, onError }) => (
+        <Script
+          key={id}
+          id={id}
+          src={src}
+          strategy={strategy}
+          onError={onError}
+          // Funnel events fired during hydration are buffered by `analytics.ts`
+          // until the vendor global exists; this is where they get drained.
+          onLoad={id === analyticsId ? flushAnalyticsQueue : undefined}
+          {...attributes}
+        />
+      ))}
     </>
   );
 }
diff --git a/src/lib/thirdParty.ts b/src/lib/thirdParty.ts
index 4de3147e..d66bee21 100644
--- a/src/lib/thirdParty.ts
+++ b/src/lib/thirdParty.ts
@@ -33,6 +33,8 @@ export interface ThirdPartyScript {
   strategy: ScriptStrategy;
   /** Extra DOM attributes, e.g. Plausible's `data-domain`. */
   attributes?: Record<string, string>;
+  /** Called when the script fails to load. */
+  onError?: () => void;
 }
 
 /** Supported privacy-first analytics vendors. */
@@ -124,10 +126,16 @@ export function getSupportWidgetScript(): ThirdPartyScript | null {
   };
 }
 
-/** Every configured third-party script, in load order. */
+/** Every configured third-party script, in load order (deduplicated by id). */
 export function getThirdPartyScripts(): ThirdPartyScript[] {
+  const seen = new Set<string>();
   return [getAnalyticsScript(), getSupportWidgetScript()].filter(
-    (script): script is ThirdPartyScript => script !== null,
+    (script): script is ThirdPartyScript => {
+      if (script === null) return false;
+      if (seen.has(script.id)) return false;
+      seen.add(script.id);
+      return true;
+    },
   );
 }
 
@@ -155,3 +163,8 @@ export function getThirdPartyScriptOrigins(): string[] {
 export function getAnalyticsProvider(): AnalyticsProvider | null {
   return getAnalyticsScript() ? readProvider() : null;
 }
+
+/** Invoke a script's error handler, if one is configured. */
+export function handleScriptError(script: ThirdPartyScript): void {
+  script.onError?.();
+}