diff --git a/app/(tools)/url-encoder/UrlEncoderClient.test.tsx b/app/(tools)/url-encoder/UrlEncoderClient.test.tsx
index ddd58c0..b99f878 100644
--- a/app/(tools)/url-encoder/UrlEncoderClient.test.tsx
+++ b/app/(tools)/url-encoder/UrlEncoderClient.test.tsx
@@ -110,4 +110,45 @@ describe("UrlEncoderClient tabs", () => {
     expect(screen.queryByRole("button", { name: "Copy encoded output" })).not.toBeInTheDocument();
     expect(screen.getByRole("button", { name: "Copy decoded output" })).toHaveTextContent("Copy");
   });
+
+  it.each([
+    ["?tag=react&tag=typescript", ["react", "typescript"]],
+    ["?tag=react&tag=react", ["react", "react"]],
+    ["?tag=&tag=typescript", ["", "typescript"]],
+    ["?tag=a&%74ag=b", ["a", "b"]],
+  ])("flags repeated parameters in %s", (query, values) => {
+    render(<UrlEncoderClient />);
+    fireEvent.change(screen.getByRole("textbox", { name: "Text or URL to encode" }), {
+      target: { value: `https://example.com/${query}` },
+    });
+    expect(screen.getAllByText("Repeated key")).toHaveLength(2);
+    expect(screen.getByRole("status")).toHaveClass("border-[var(--amber)]", "text-[var(--amber)]");
+    expect(screen.getByRole("status")).toHaveTextContent("1 repeated key: tag.");
+    expect(screen.getAllByText("Repeated key")[0]).toHaveClass("border-[var(--amber)]", "text-[var(--amber)]");
+    expect(screen.getByText(/not necessarily errors/)).toBeInTheDocument();
+    const rows = screen.getAllByRole("button", { name: "Copy value of tag" });
+    expect(rows.map((button) => button.parentElement?.querySelector("span.text-\\[var\\(--fg\\)\\]")?.textContent)).toEqual(values);
+  });
+
+  it("keeps case-sensitive and unique keys unflagged and labels empty names", () => {
+    render(<UrlEncoderClient />);
+    const input = screen.getByRole("textbox", { name: "Text or URL to encode" });
+    fireEvent.change(input, { target: { value: "https://example.com/?Tag=a&tag=b&=empty" } });
+    expect(screen.queryByText("Repeated key")).not.toBeInTheDocument();
+    expect(screen.queryByRole("status")).not.toBeInTheDocument();
+    expect(screen.getByRole("button", { name: "Copy value of (empty name)" }).parentElement).toHaveTextContent("(empty name) = empty");
+    fireEvent.change(input, { target: { value: "https://example.com/?=first&=second" } });
+    expect(screen.getAllByText("Repeated key")).toHaveLength(2);
+    expect(screen.getByRole("status")).toHaveTextContent("1 repeated key: (empty name).");
+  });
+
+  it("updates repeated-key indicators when the URL changes", () => {
+    render(<UrlEncoderClient />);
+    const input = screen.getByRole("textbox", { name: "Text or URL to encode" });
+    fireEvent.change(input, { target: { value: "https://example.com/?tag=a&tag=b" } });
+    expect(screen.getAllByText("Repeated key")).toHaveLength(2);
+    fireEvent.change(input, { target: { value: "https://example.com/?tag=a&sort=b" } });
+    expect(screen.queryByText("Repeated key")).not.toBeInTheDocument();
+    expect(screen.queryByRole("status")).not.toBeInTheDocument();
+  });
 });
diff --git a/app/(tools)/url-encoder/UrlEncoderClient.tsx b/app/(tools)/url-encoder/UrlEncoderClient.tsx
index 8150c77..f4f1deb 100644
--- a/app/(tools)/url-encoder/UrlEncoderClient.tsx
+++ b/app/(tools)/url-encoder/UrlEncoderClient.tsx
@@ -2,7 +2,7 @@
 
 import { useMemo, useRef, useState } from "react";
 import { AlertCircle, ArrowLeftRight } from "lucide-react";
-import { encodeUrl, decodeUrl, parseUrl, type UrlEncodeMode } from "@/lib/url-utils";
+import { analyzeQueryParams, encodeUrl, decodeUrl, parseUrl, type UrlEncodeMode } from "@/lib/url-utils";
 import { CopyButton } from "@/components/shared/CopyButton";
 import { useRegisterCommands } from "@/components/shared/CommandPalette";
 
@@ -40,6 +40,7 @@ export function UrlEncoderClient() {
     }
     return null;
   }, [input, output]);
+  const queryParams = useMemo(() => analyzeQueryParams(parsed?.params ?? []), [parsed]);
 
   function onTabKeyDown(e: React.KeyboardEvent, index: number) {
     let next = index;
@@ -211,14 +212,20 @@ export function UrlEncoderClient() {
               {parsed.hash && <Row label="Hash" value={parsed.hash} />}
               {parsed.params.length > 0 && (
                 <div className="pt-2 mt-1 border-t border-[var(--hairline)] space-y-1.5">
-                  {parsed.params.map(([key, value], i) => (
+                  {queryParams.repeatedKeys.length > 0 && (
+                    <div role="status" className="rounded-lg border border-[var(--amber)] p-2 text-[var(--amber)]">
+                      {queryParams.repeatedKeys.length} repeated {queryParams.repeatedKeys.length === 1 ? "key" : "keys"}: {queryParams.repeatedKeys.map((key) => key || "(empty name)").join(", ")}. Repeated parameters are not necessarily errors; applications may interpret them differently.
+                    </div>
+                  )}
+                  {queryParams.occurrences.map(({ key, value, repeated }, i) => (
                     <div key={`${key}-${i}`} className="flex items-center justify-between gap-2">
                       <span className="text-[var(--fg-muted)] break-all">
-                        {key} = <span className="text-[var(--fg)]">{value}</span>
+                        {key || "(empty name)"} = <span className="text-[var(--fg)]">{value}</span>
+                        {repeated && <span className="ml-2 inline-block rounded border border-[var(--amber)] px-1.5 py-0.5 text-[10px] font-sans font-semibold text-[var(--amber)]">Repeated key</span>}
                       </span>
                       <CopyButton
                         value={value}
-                        ariaLabel={`Copy value of ${key}`}
+                        ariaLabel={`Copy value of ${key || "(empty name)"}`}
                         className="py-0.5 shrink-0"
                       />
                     </div>
diff --git a/lib/__tests__/url-utils.test.ts b/lib/__tests__/url-utils.test.ts
index 779634c..81ad619 100644
--- a/lib/__tests__/url-utils.test.ts
+++ b/lib/__tests__/url-utils.test.ts
@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { encodeUrl, decodeUrl, parseUrl } from "../url-utils";
+import { analyzeQueryParams, encodeUrl, decodeUrl, parseUrl } from "../url-utils";
 
 describe("encodeUrl / decodeUrl round-trip", () => {
   it("round-trips unicode text", () => {
@@ -81,3 +81,24 @@ describe("parseUrl", () => {
     expect(r.ok).toBe(false);
   });
 });
+
+describe("analyzeQueryParams", () => {
+  it("marks every same-valued and different-valued repeat and reports distinct repeated names", () => {
+    const result = analyzeQueryParams([["tag", "react"], ["tag", "typescript"], ["sort", "a"], ["sort", "a"]]);
+    expect(result.occurrences.map(({ repeated }) => repeated)).toEqual([true, true, true, true]);
+    expect(result.repeatedKeys).toEqual(["tag", "sort"]);
+  });
+  it("groups decoded names, preserves case sensitivity, empty names, values, and order", () => {
+    const result = analyzeQueryParams([["tag", "a"], ["tag", "b"], ["Tag", "c"], ["tag", ""], ["", ""], ["", "x"]]);
+    expect(result.occurrences).toEqual([
+      { key: "tag", value: "a", repeated: true }, { key: "tag", value: "b", repeated: true },
+      { key: "Tag", value: "c", repeated: false }, { key: "tag", value: "", repeated: true },
+      { key: "", value: "", repeated: true }, { key: "", value: "x", repeated: true },
+    ]);
+    expect(result.repeatedKeys).toEqual(["tag", ""]);
+  });
+  it("leaves unique and empty parameter lists unflagged", () => {
+    expect(analyzeQueryParams([["a", ""], ["b", "2"]]).repeatedKeys).toEqual([]);
+    expect(analyzeQueryParams([])).toEqual({ occurrences: [], repeatedKeys: [] });
+  });
+});
diff --git a/lib/url-utils.ts b/lib/url-utils.ts
index 72f7ff1..53c7aba 100644
--- a/lib/url-utils.ts
+++ b/lib/url-utils.ts
@@ -37,6 +37,26 @@ export interface ParsedUrl {
   params: [string, string][];
 }
 
+export interface QueryParamOccurrence {
+  key: string;
+  value: string;
+  repeated: boolean;
+}
+
+/** Marks every occurrence of each repeated decoded name without changing order or values. */
+export function analyzeQueryParams(params: readonly [string, string][]): {
+  occurrences: QueryParamOccurrence[];
+  repeatedKeys: string[];
+} {
+  const counts = new Map<string, number>();
+  for (const [key] of params) counts.set(key, (counts.get(key) ?? 0) + 1);
+  const repeatedKeys = [...counts].filter(([, count]) => count > 1).map(([key]) => key);
+  return {
+    occurrences: params.map(([key, value]) => ({ key, value, repeated: counts.get(key)! > 1 })),
+    repeatedKeys,
+  };
+}
+
 export function parseUrl(text: string): { ok: true; value: ParsedUrl } | { ok: false; error: string } {
   try {
     const url = new URL(text);