diff --git a/src/Lean/Linter/EnvLinter/Builtin.lean b/src/Lean/Linter/EnvLinter/Builtin.lean
index 565599d449..068149b298 100644
--- a/src/Lean/Linter/EnvLinter/Builtin.lean
+++ b/src/Lean/Linter/EnvLinter/Builtin.lean
@@ -97,6 +97,18 @@ a higher universe level than `α`. -/
     if bad.isEmpty then return none
     return m!"universes {bad} only occur together."
 
+/-- A linter for checking whether a declaration has a namespace twice consecutively in its name. -/
+@[builtin_env_linter clippy] def dupNamespace : EnvLinter where
+  noErrorsFound := "No declarations have a duplicate namespace."
+  errorsFound := "DUPLICATED NAMESPACES IN NAME:"
+  test declName := do
+    if ← isAutoDecl declName then return none
+    if ← isImplicitReducible declName then return none
+    let nm := declName.components
+    let some (dup, _) := nm.zip nm.tail! |>.find? fun (x, y) => x == y
+      | return none
+    return m!"The namespace {dup} is duplicated in the name"
+
 end Lean.Linter.EnvLinter
 
 end
diff --git a/tests/elab/env_linter.lean b/tests/elab/env_linter.lean
index 36589659b1..0c5b5202de 100644
--- a/tests/elab/env_linter.lean
+++ b/tests/elab/env_linter.lean
@@ -140,7 +140,7 @@ def testGetChecksClippy : CoreM (Array Name) := do
   let checks ← getChecks (scope := .clippy) (runOnly := none)
   return checks.map (·.name)
 
-/-- info: #[`dummyClippyLinter] -/
+/-- info: #[`dummyClippyLinter, `dupNamespace] -/
 #guard_msgs in
 #eval testGetChecksClippy
 
@@ -149,7 +149,7 @@ def testGetChecksAll : CoreM (Array Name) := do
   let checks ← getChecks (scope := .all) (runOnly := none)
   return checks.map (·.name)
 
-/-- info: #[`checkUnivs, `defLemma, `dummyBadName, `dummyClippyLinter] -/
+/-- info: #[`checkUnivs, `defLemma, `dummyBadName, `dummyClippyLinter, `dupNamespace] -/
 #guard_msgs in
 #eval testGetChecksAll
 
diff --git a/tests/lake/tests/builtin-lint/ClippyViolations.lean b/tests/lake/tests/builtin-lint/ClippyViolations.lean
index b4e9c015e7..0a9438bc5a 100644
--- a/tests/lake/tests/builtin-lint/ClippyViolations.lean
+++ b/tests/lake/tests/builtin-lint/ClippyViolations.lean
@@ -2,3 +2,7 @@ import Linters
 
 -- This name ends with 'Clippy' — the dummyClippy linter should flag it.
 def badNameClippy : Nat := 1
+
+-- The component `Dup` appears consecutively in this declaration's name —
+-- the builtin clippy `dupNamespace` env linter should flag it.
+def Dup.Dup.violation : Nat := 2
diff --git a/tests/lake/tests/builtin-lint/input/ClippyViolations.lean b/tests/lake/tests/builtin-lint/input/ClippyViolations.lean
index b4e9c015e7..0a9438bc5a 100644
--- a/tests/lake/tests/builtin-lint/input/ClippyViolations.lean
+++ b/tests/lake/tests/builtin-lint/input/ClippyViolations.lean
@@ -2,3 +2,7 @@ import Linters
 
 -- This name ends with 'Clippy' — the dummyClippy linter should flag it.
 def badNameClippy : Nat := 1
+
+-- The component `Dup` appears consecutively in this declaration's name —
+-- the builtin clippy `dupNamespace` env linter should flag it.
+def Dup.Dup.violation : Nat := 2
diff --git a/tests/lake/tests/builtin-lint/test.sh b/tests/lake/tests/builtin-lint/test.sh
index 0fd16aab37..3c25949692 100755
--- a/tests/lake/tests/builtin-lint/test.sh
+++ b/tests/lake/tests/builtin-lint/test.sh
@@ -85,6 +85,8 @@ test_run lint --builtin-only Clean
 lake_out lint --builtin-only ClippyViolations || true
 no_match_pat 'badNameClippy' produced.out
 no_match_pat 'clippy text linter saw a declaration' produced.out
+# Builtin clippy env linter `dupNamespace` is non-default, so it stays silent.
+no_match_pat 'Dup.Dup.violation' produced.out
 
 # --clippy should run only non-default (clippy) linters, including the clippy
 # text linter which tags its warnings with `linter.clippy`.
@@ -92,6 +94,9 @@ lake_out lint --clippy ClippyViolations || true
 match_pat 'badNameClippy' produced.out
 match_pat "declaration name ends with 'Clippy'" produced.out
 match_pat 'clippy text linter saw a declaration' produced.out
+# Builtin `dupNamespace` env linter fires under --clippy.
+match_pat 'Dup.Dup.violation' produced.out
+match_pat "namespace .*Dup.* is duplicated" produced.out
 # --clippy should not run default linters
 no_match_pat 'shouldBeTheorem' produced.out
 
@@ -108,6 +113,13 @@ no_match_pat 'missing doc string' produced.out
 lake_out lint --lint-all ClippyViolations || true
 match_pat 'badNameClippy' produced.out
 match_pat 'clippy text linter saw a declaration' produced.out
+match_pat 'Dup.Dup.violation' produced.out
+
+# --lint-only dupNamespace runs only the builtin clippy `dupNamespace` env linter.
+lake_out lint --lint-only dupNamespace ClippyViolations || true
+match_pat 'Dup.Dup.violation' produced.out
+no_match_pat 'badNameClippy' produced.out
+no_match_pat 'shouldBeTheorem' produced.out
 
 # Multiple --lint-only flags accumulate: both named linters should run
 lake_out lint --lint-only defLemma --lint-only checkUnivs || true