diff --git a/packages/next/next-server/lib/router/router.ts b/packages/next/next-server/lib/router/router.ts
index 785cbf0bbaf8b..45379d17937d0 100644
--- a/packages/next/next-server/lib/router/router.ts
+++ b/packages/next/next-server/lib/router/router.ts
@@ -113,7 +113,8 @@ export function addLocale(
   defaultLocale?: string
 ) {
   if (process.env.__NEXT_I18N_SUPPORT) {
-    const pathLower = path.toLowerCase()
+    const pathname = pathNoQueryHash(path)
+    const pathLower = pathname.toLowerCase()
     const localeLower = locale && locale.toLowerCase()
 
     return locale &&
diff --git a/packages/next/package.json b/packages/next/package.json
index 23e3436d1c1b2..366c470a826e9 100644
--- a/packages/next/package.json
+++ b/packages/next/package.json
@@ -28,6 +28,8 @@
     "dynamic.d.ts",
     "error.js",
     "error.d.ts",
+    "experimental-script.js",
+    "experimental-script.d.ts",
     "head.js",
     "head.d.ts",
     "image.js",
diff --git a/test/integration/i18n-support/test/shared.js b/test/integration/i18n-support/test/shared.js
index f183ee1b9dc0b..f0dac7dafbad3 100644
--- a/test/integration/i18n-support/test/shared.js
+++ b/test/integration/i18n-support/test/shared.js
@@ -53,6 +53,84 @@ export function runTests(ctx) {
     expect(await res.text()).toContain('index page')
   })
 
+  it('should not add duplicate locale key when navigating back to root path with query params', async () => {
+    const basePath = ctx.basePath || ''
+    const queryKey = 'query'
+    const queryValue = '1'
+    const browser = await webdriver(
+      ctx.appPort,
+      `${basePath}/fr?${queryKey}=${queryValue}`
+    )
+
+    expect(await browser.eval(() => document.location.pathname)).toBe(
+      `${basePath}/fr`
+    )
+    expect(await browser.elementByCss('#router-pathname').text()).toBe('/')
+    expect(
+      JSON.parse(await browser.elementByCss('#router-query').text())
+    ).toEqual({ [queryKey]: queryValue })
+    expect(await browser.elementByCss('#router-locale').text()).toBe('fr')
+
+    await browser
+      .elementByCss('#to-another')
+      .click()
+      .waitForElementByCss('#another')
+
+    expect(await browser.eval(() => document.location.pathname)).toBe(
+      `${basePath}/fr/another`
+    )
+    expect(await browser.elementByCss('#router-pathname').text()).toBe(
+      '/another'
+    )
+    expect(await browser.elementByCss('#router-locale').text()).toBe('fr')
+
+    await browser.back().waitForElementByCss('#index')
+
+    expect(await browser.eval(() => document.location.pathname)).toBe(
+      `${basePath}/fr`
+    )
+    expect(await browser.elementByCss('#router-pathname').text()).toBe('/')
+    expect(
+      JSON.parse(await browser.elementByCss('#router-query').text())
+    ).toEqual({ [queryKey]: queryValue })
+    expect(await browser.elementByCss('#router-locale').text()).toBe('fr')
+  })
+
+  it('should not add duplicate locale key when navigating back to root path with hash', async () => {
+    const basePath = ctx.basePath || ''
+    const hashValue = '#anchor-1'
+    const browser = await webdriver(ctx.appPort, `${basePath}/fr${hashValue}`)
+
+    expect(await browser.eval(() => document.location.pathname)).toBe(
+      `${basePath}/fr`
+    )
+    expect(await browser.eval(() => document.location.hash)).toBe(hashValue)
+    expect(await browser.elementByCss('#router-pathname').text()).toBe('/')
+    expect(await browser.elementByCss('#router-locale').text()).toBe('fr')
+
+    await browser
+      .elementByCss('#to-another')
+      .click()
+      .waitForElementByCss('#another')
+
+    expect(await browser.eval(() => document.location.pathname)).toBe(
+      `${basePath}/fr/another`
+    )
+    expect(await browser.elementByCss('#router-pathname').text()).toBe(
+      '/another'
+    )
+    expect(await browser.elementByCss('#router-locale').text()).toBe('fr')
+
+    await browser.back().waitForElementByCss('#index')
+
+    expect(await browser.eval(() => document.location.pathname)).toBe(
+      `${basePath}/fr`
+    )
+    expect(await browser.eval(() => document.location.hash)).toBe(hashValue)
+    expect(await browser.elementByCss('#router-pathname').text()).toBe('/')
+    expect(await browser.elementByCss('#router-locale').text()).toBe('fr')
+  })
+
   it('should handle navigating back to different casing of locale', async () => {
     const browser = await webdriver(
       ctx.appPort,