diff --git a/CHANGELOG.md b/CHANGELOG.md
index a0bc89c..d7d4ee2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,10 +6,12 @@ Changes are tagged: **[wrapper]** for Python/JS wrapper, **[binary]** for Chromi
 
 ---
 
-## [0.3.8] — 2026-03-05
+## [0.3.9] — 2026-03-05
 
 - **[binary]** Upgrade Chromium base to 145.0.7632.159 (Linux x64). macOS and Windows remain on 145.0.7632.109.2
 - **[binary]** WebGPU adapter spoofing for headless/Docker, timezone multi-context fix, stealth audit phase 2 (6 detection vector fixes), font auto-hide for cross-platform fingerprints
+- **[wrapper]** Deduplicate CLI flags when user args overlap with stealth defaults — user values win cleanly instead of passing both to Chromium
+- **[wrapper]** Extract shared `buildArgs` into `js/src/args.ts` (JS DRY fix), guard debug logging behind `DEBUG=cloakbrowser` env var
 
 ## [0.3.7] — 2026-03-05
 
diff --git a/cloakbrowser/_version.py b/cloakbrowser/_version.py
index 4ad67eb..771bc6e 100644
--- a/cloakbrowser/_version.py
+++ b/cloakbrowser/_version.py
@@ -1 +1 @@
-__version__ = "0.3.8"
+__version__ = "0.3.9"
diff --git a/cloakbrowser/browser.py b/cloakbrowser/browser.py
index a081a27..44221c7 100644
--- a/cloakbrowser/browser.py
+++ b/cloakbrowser/browser.py
@@ -496,18 +496,39 @@ def _build_args(
     timezone: str | None = None,
     locale: str | None = None,
 ) -> list[str]:
-    """Combine stealth args with user-provided args and locale flags."""
-    result = []
+    """Combine stealth args with user-provided args and locale flags.
+
+    Deduplicates by flag key (everything before '=').
+    Priority: stealth defaults < user args < dedicated params (timezone/locale).
+    """
+    seen: dict[str, str] = {}
+
     if stealth_args:
-        result.extend(get_default_stealth_args())
+        for arg in get_default_stealth_args():
+            seen[arg.split("=", 1)[0]] = arg
+
     if extra_args:
-        result.extend(extra_args)
+        for arg in extra_args:
+            key = arg.split("=", 1)[0]
+            if key in seen:
+                logger.debug("Arg override: %s -> %s", seen[key], arg)
+            seen[key] = arg
+
     # Timezone/locale flags are independent of stealth_args — always inject when set
     if timezone:
-        result.append(f"--fingerprint-timezone={timezone}")
+        key = "--fingerprint-timezone"
+        flag = f"{key}={timezone}"
+        if key in seen:
+            logger.debug("Arg override: %s -> %s", seen[key], flag)
+        seen[key] = flag
     if locale:
-        result.append(f"--lang={locale}")
-    return result
+        key = "--lang"
+        flag = f"{key}={locale}"
+        if key in seen:
+            logger.debug("Arg override: %s -> %s", seen[key], flag)
+        seen[key] = flag
+
+    return list(seen.values())
 
 
 def _parse_proxy_url(proxy: str) -> dict[str, Any]:
diff --git a/js/package.json b/js/package.json
index e20713f..6e820be 100644
--- a/js/package.json
+++ b/js/package.json
@@ -1,6 +1,6 @@
 {
   "name": "cloakbrowser",
-  "version": "0.3.8",
+  "version": "0.3.9",
   "description": "Stealth Chromium that passes every bot detection test. Drop-in Playwright/Puppeteer replacement with source-level fingerprint patches.",
   "type": "module",
   "main": "dist/index.js",
diff --git a/js/src/args.ts b/js/src/args.ts
new file mode 100644
index 0000000..0df410a
--- /dev/null
+++ b/js/src/args.ts
@@ -0,0 +1,49 @@
+/**
+ * Shared argument builder for Playwright and Puppeteer wrappers.
+ */
+
+import type { LaunchOptions } from "./types.js";
+import { getDefaultStealthArgs } from "./config.js";
+
+const DEBUG = /\bcloakbrowser\b/.test(process.env.DEBUG ?? "");
+
+/**
+ * Build deduplicated Chromium CLI args from stealth defaults + user overrides.
+ *
+ * Priority: stealth defaults < user args < dedicated params (timezone/locale).
+ */
+export function buildArgs(options: LaunchOptions): string[] {
+  const seen = new Map<string, string>();
+
+  if (options.stealthArgs !== false) {
+    for (const arg of getDefaultStealthArgs()) {
+      seen.set(arg.split("=")[0], arg);
+    }
+  }
+  if (options.args) {
+    for (const arg of options.args) {
+      const key = arg.split("=")[0];
+      if (seen.has(key)) {
+        if (DEBUG) console.debug(`[cloakbrowser] Arg override: ${seen.get(key)} -> ${arg}`);
+      }
+      seen.set(key, arg);
+    }
+  }
+  if (options.timezone) {
+    const key = "--fingerprint-timezone";
+    const flag = `${key}=${options.timezone}`;
+    if (seen.has(key)) {
+      if (DEBUG) console.debug(`[cloakbrowser] Arg override: ${seen.get(key)} -> ${flag}`);
+    }
+    seen.set(key, flag);
+  }
+  if (options.locale) {
+    const key = "--lang";
+    const flag = `${key}=${options.locale}`;
+    if (seen.has(key)) {
+      if (DEBUG) console.debug(`[cloakbrowser] Arg override: ${seen.get(key)} -> ${flag}`);
+    }
+    seen.set(key, flag);
+  }
+  return [...seen.values()];
+}
diff --git a/js/src/playwright.ts b/js/src/playwright.ts
index 3eecd8f..3c57461 100644
--- a/js/src/playwright.ts
+++ b/js/src/playwright.ts
@@ -5,7 +5,8 @@
 
 import type { Browser, BrowserContext } from "playwright-core";
 import type { LaunchOptions, LaunchContextOptions, LaunchPersistentContextOptions } from "./types.js";
-import { DEFAULT_VIEWPORT, getDefaultStealthArgs } from "./config.js";
+import { DEFAULT_VIEWPORT } from "./config.js";
+import { buildArgs } from "./args.js";
 import { ensureBinary } from "./download.js";
 import { parseProxyUrl } from "./proxy.js";
 
@@ -176,24 +177,4 @@ async function maybeResolveGeoip(
 }
 
 /** @internal Exposed for unit tests only. */
-export function _buildArgsForTest(options: LaunchOptions): string[] {
-  return buildArgs(options);
-}
-
-function buildArgs(options: LaunchOptions): string[] {
-  const args: string[] = [];
-  if (options.stealthArgs !== false) {
-    args.push(...getDefaultStealthArgs());
-  }
-  if (options.args) {
-    args.push(...options.args);
-  }
-  // Timezone/locale flags — always inject when set
-  if (options.timezone) {
-    args.push(`--fingerprint-timezone=${options.timezone}`);
-  }
-  if (options.locale) {
-    args.push(`--lang=${options.locale}`);
-  }
-  return args;
-}
+export { buildArgs as _buildArgsForTest } from "./args.js";
diff --git a/js/src/puppeteer.ts b/js/src/puppeteer.ts
index 4ff917f..15a596e 100644
--- a/js/src/puppeteer.ts
+++ b/js/src/puppeteer.ts
@@ -5,7 +5,7 @@
 
 import type { Browser } from "puppeteer-core";
 import type { LaunchOptions } from "./types.js";
-import { getDefaultStealthArgs } from "./config.js";
+import { buildArgs } from "./args.js";
 import { ensureBinary } from "./download.js";
 import { parseProxyUrl } from "./proxy.js";
 
@@ -99,19 +99,3 @@ async function maybeResolveGeoip(
   };
 }
 
-function buildArgs(options: LaunchOptions): string[] {
-  const args: string[] = [];
-  if (options.stealthArgs !== false) {
-    args.push(...getDefaultStealthArgs());
-  }
-  if (options.args) {
-    args.push(...options.args);
-  }
-  if (options.timezone) {
-    args.push(`--fingerprint-timezone=${options.timezone}`);
-  }
-  if (options.locale) {
-    args.push(`--lang=${options.locale}`);
-  }
-  return args;
-}
diff --git a/js/tests/config.test.ts b/js/tests/config.test.ts
index fb013f6..a79e011 100644
--- a/js/tests/config.test.ts
+++ b/js/tests/config.test.ts
@@ -123,6 +123,59 @@ describe("buildArgs timezone/locale", () => {
   });
 });
 
+describe("buildArgs deduplication", () => {
+  it("user --fingerprint overrides default seed", () => {
+    const args = _buildArgsForTest({ args: ["--fingerprint=99887"] });
+    const fpArgs = args.filter(a => a.startsWith("--fingerprint="));
+    expect(fpArgs).toHaveLength(1);
+    expect(fpArgs[0]).toBe("--fingerprint=99887");
+  });
+
+  it("user --fingerprint-platform overrides default", () => {
+    const args = _buildArgsForTest({ args: ["--fingerprint-platform=linux"] });
+    const platArgs = args.filter(a => a.startsWith("--fingerprint-platform="));
+    expect(platArgs).toHaveLength(1);
+    expect(platArgs[0]).toBe("--fingerprint-platform=linux");
+  });
+
+  it("timezone param overrides user --fingerprint-timezone arg", () => {
+    const args = _buildArgsForTest({
+      args: ["--fingerprint-timezone=Europe/London"],
+      timezone: "America/New_York",
+    });
+    const tzArgs = args.filter(a => a.startsWith("--fingerprint-timezone="));
+    expect(tzArgs).toHaveLength(1);
+    expect(tzArgs[0]).toBe("--fingerprint-timezone=America/New_York");
+  });
+
+  it("locale param overrides user --lang arg", () => {
+    const args = _buildArgsForTest({
+      args: ["--lang=de-DE"],
+      locale: "en-US",
+    });
+    const langArgs = args.filter(a => a.startsWith("--lang="));
+    expect(langArgs).toHaveLength(1);
+    expect(langArgs[0]).toBe("--lang=en-US");
+  });
+
+  it("no duplicate flag keys in output", () => {
+    const args = _buildArgsForTest({
+      args: ["--fingerprint=99887", "--fingerprint-timezone=UTC", "--lang=fr-FR"],
+      timezone: "Europe/Berlin",
+      locale: "de-DE",
+    });
+    const keys = args.map(a => a.split("=")[0]);
+    expect(new Set(keys).size).toBe(keys.length);
+  });
+
+  it("non-value flags preserved without dedup issues", () => {
+    const args = _buildArgsForTest({ args: ["--disable-gpu", "--no-zygote"] });
+    expect(args).toContain("--disable-gpu");
+    expect(args).toContain("--no-zygote");
+    expect(args).toContain("--no-sandbox");
+  });
+});
+
 describe("migrateTimezoneId deprecation", () => {
   it("migrates timezoneId to timezone", () => {
     const result = migrateTimezoneId({ timezoneId: "Europe/Paris" });
diff --git a/tests/test_build_args.py b/tests/test_build_args.py
index 201c671..9ab588f 100644
--- a/tests/test_build_args.py
+++ b/tests/test_build_args.py
@@ -92,3 +92,75 @@ def test_migrate_both_none():
         result = _migrate_timezone_id(None, kwargs)
     assert result is None
     assert len(w) == 0
+
+
+# --- Deduplication tests ---
+
+
+def test_user_fingerprint_overrides_default():
+    """User --fingerprint should override the random default seed."""
+    args = _build_args(stealth_args=True, extra_args=["--fingerprint=99887"])
+    fingerprint_args = [a for a in args if a.startswith("--fingerprint=")]
+    assert len(fingerprint_args) == 1
+    assert fingerprint_args[0] == "--fingerprint=99887"
+
+
+def test_user_platform_overrides_default():
+    """User --fingerprint-platform should override the default."""
+    args = _build_args(stealth_args=True, extra_args=["--fingerprint-platform=linux"])
+    platform_args = [a for a in args if a.startswith("--fingerprint-platform=")]
+    assert len(platform_args) == 1
+    assert platform_args[0] == "--fingerprint-platform=linux"
+
+
+def test_timezone_param_overrides_user_arg():
+    """Dedicated timezone param should override user arg."""
+    args = _build_args(
+        stealth_args=True,
+        extra_args=["--fingerprint-timezone=Europe/London"],
+        timezone="America/New_York",
+    )
+    tz_args = [a for a in args if a.startswith("--fingerprint-timezone=")]
+    assert len(tz_args) == 1
+    assert tz_args[0] == "--fingerprint-timezone=America/New_York"
+
+
+def test_locale_param_overrides_user_arg():
+    """Dedicated locale param should override user --lang arg."""
+    args = _build_args(
+        stealth_args=True,
+        extra_args=["--lang=de-DE"],
+        locale="en-US",
+    )
+    lang_args = [a for a in args if a.startswith("--lang=")]
+    assert len(lang_args) == 1
+    assert lang_args[0] == "--lang=en-US"
+
+
+def test_no_duplicate_flags():
+    """No flag key should appear more than once in the output."""
+    args = _build_args(
+        stealth_args=True,
+        extra_args=["--fingerprint=99887", "--fingerprint-timezone=UTC", "--lang=fr-FR"],
+        timezone="Europe/Berlin",
+        locale="de-DE",
+    )
+    keys = [a.split("=", 1)[0] for a in args]
+    assert len(keys) == len(set(keys)), f"Duplicate keys found: {keys}"
+
+
+def test_non_value_flags_preserved():
+    """Flags without = should be preserved without dedup issues."""
+    args = _build_args(stealth_args=True, extra_args=["--disable-gpu", "--no-zygote"])
+    assert "--disable-gpu" in args
+    assert "--no-zygote" in args
+    assert "--no-sandbox" in args
+
+
+def test_override_logs_debug(caplog):
+    """Should log debug message when an override happens."""
+    import logging
+
+    with caplog.at_level(logging.DEBUG, logger="cloakbrowser"):
+        _build_args(stealth_args=True, extra_args=["--fingerprint=99887"])
+    assert any("--fingerprint=" in r.message and "99887" in r.message for r in caplog.records)