diff --git a/compiler/rustc_ty_utils/src/assoc.rs b/compiler/rustc_ty_utils/src/assoc.rs
index a65f9b347dc6d..2fb3c5ff945af 100644
--- a/compiler/rustc_ty_utils/src/assoc.rs
+++ b/compiler/rustc_ty_utils/src/assoc.rs
@@ -195,12 +195,39 @@ fn associated_types_for_impl_traits_in_associated_fn(
     match tcx.def_kind(parent_def_id) {
         DefKind::Trait => {
             if let Some(output) = tcx.hir_get_fn_output(fn_def_id) {
-                let data = DefPathData::AnonAssocTy(tcx.item_name(fn_def_id.to_def_id()));
+                let def_path_id = |def_id: LocalDefId| tcx.item_name(def_id.to_def_id());
+                let def_path_data = def_path_id(fn_def_id);
+
+                let (.., trait_item_refs) = tcx.hir_expect_item(parent_def_id).expect_trait();
+                // The purpose of `disambiguator_idx` is to ensure there are
+                // no duplicate `def_id` in certain cases, such as:
+                // ```
+                // trait Foo {
+                //     fn bar() -> impl Trait;
+                //     fn bar() -> impl Trait;
+                //              // ~~~~~~~~~~ It will generate the same ID if we don’t disambiguate it.
+                // }
+                // ```
+                let disambiguator_idx = trait_item_refs
+                    .iter()
+                    .take_while(|item| item.id.owner_id.def_id != fn_def_id)
+                    .fold(0, |acc, item| {
+                        if !matches!(item.kind, hir::AssocItemKind::Fn { .. }) {
+                            acc
+                        } else if def_path_id(item.id.owner_id.def_id) == def_path_data {
+                            tcx.def_key(item.id.owner_id.def_id).disambiguated_data.disambiguator
+                                + 1
+                        } else {
+                            acc
+                        }
+                    });
+
+                let data = DefPathData::AnonAssocTy(def_path_data);
                 let mut visitor = RPITVisitor {
                     tcx,
                     synthetics: vec![],
                     data,
-                    disambiguator: DisambiguatorState::with(parent_def_id, data, 0),
+                    disambiguator: DisambiguatorState::with(parent_def_id, data, disambiguator_idx),
                 };
                 visitor.visit_fn_ret_ty(output);
                 tcx.arena.alloc_from_iter(
diff --git a/tests/ui/impl-trait/in-trait/rpitit-duplicate-associated-fn.rs b/tests/ui/impl-trait/in-trait/rpitit-duplicate-associated-fn.rs
new file mode 100644
index 0000000000000..4fddd7c4ac8c6
--- /dev/null
+++ b/tests/ui/impl-trait/in-trait/rpitit-duplicate-associated-fn.rs
@@ -0,0 +1,30 @@
+// issue#140796
+
+trait Bar {
+    fn method() -> impl Sized;
+    fn method() -> impl Sized;  //~ ERROR: the name `method` is defined multiple times
+}
+
+impl Bar for () {               //~ ERROR: not all trait items implemented, missing: `method`
+    fn method() -> impl Sized {
+        42
+    }
+    fn method() -> impl Sized { //~ ERROR: duplicate definitions with name `method`
+        42
+    }
+}
+
+trait T {
+    fn method() -> impl Sized;
+}
+
+impl T for () {
+    fn method() -> impl Sized {
+        42
+    }
+    fn method() -> impl Sized { //~ ERROR: duplicate definitions with name `method`
+        42
+    }
+}
+
+fn main() {}
diff --git a/tests/ui/impl-trait/in-trait/rpitit-duplicate-associated-fn.stderr b/tests/ui/impl-trait/in-trait/rpitit-duplicate-associated-fn.stderr
new file mode 100644
index 0000000000000..b58e8136479c1
--- /dev/null
+++ b/tests/ui/impl-trait/in-trait/rpitit-duplicate-associated-fn.stderr
@@ -0,0 +1,53 @@
+error[E0428]: the name `method` is defined multiple times
+  --> $DIR/rpitit-duplicate-associated-fn.rs:5:5
+   |
+LL |     fn method() -> impl Sized;
+   |     -------------------------- previous definition of the value `method` here
+LL |     fn method() -> impl Sized;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ `method` redefined here
+   |
+   = note: `method` must be defined only once in the value namespace of this trait
+
+error[E0201]: duplicate definitions with name `method`:
+  --> $DIR/rpitit-duplicate-associated-fn.rs:12:5
+   |
+LL |       fn method() -> impl Sized;
+   |       -------------------------- item in trait
+...
+LL | /     fn method() -> impl Sized {
+LL | |         42
+LL | |     }
+   | |_____- previous definition here
+LL | /     fn method() -> impl Sized {
+LL | |         42
+LL | |     }
+   | |_____^ duplicate definition
+
+error[E0201]: duplicate definitions with name `method`:
+  --> $DIR/rpitit-duplicate-associated-fn.rs:25:5
+   |
+LL |       fn method() -> impl Sized;
+   |       -------------------------- item in trait
+...
+LL | /     fn method() -> impl Sized {
+LL | |         42
+LL | |     }
+   | |_____- previous definition here
+LL | /     fn method() -> impl Sized {
+LL | |         42
+LL | |     }
+   | |_____^ duplicate definition
+
+error[E0046]: not all trait items implemented, missing: `method`
+  --> $DIR/rpitit-duplicate-associated-fn.rs:8:1
+   |
+LL |     fn method() -> impl Sized;
+   |     -------------------------- `method` from trait
+...
+LL | impl Bar for () {
+   | ^^^^^^^^^^^^^^^ missing `method` in implementation
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0046, E0201, E0428.
+For more information about an error, try `rustc --explain E0046`.