diff --git a/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/duplicated-accessibility-modifiers/fixture.ts b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/duplicated-accessibility-modifiers/fixture.ts
new file mode 100644
index 000000000000..29db5140d271
--- /dev/null
+++ b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/duplicated-accessibility-modifiers/fixture.ts
@@ -0,0 +1,3 @@
+class Foo {
+  public public bar() {};
+}
diff --git a/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/1-TSESTree-Error.shot b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/1-TSESTree-Error.shot
new file mode 100644
index 000000000000..49db5284df5a
--- /dev/null
+++ b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/1-TSESTree-Error.shot
@@ -0,0 +1,10 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`AST Fixtures element MethodDefinition _error_ duplicated-accessibility-modifiers TSESTree - Error 1`] = `
+"TSError
+  1 | class Foo {
+> 2 |   public public bar() {};
+    |          ^^^^^^ Accessibility modifier already seen.
+  3 | }
+  4 |"
+`;
diff --git a/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/2-Babel-Error.shot b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/2-Babel-Error.shot
new file mode 100644
index 000000000000..e84a8adf3e77
--- /dev/null
+++ b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/2-Babel-Error.shot
@@ -0,0 +1,3 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`AST Fixtures element MethodDefinition _error_ duplicated-accessibility-modifiers Babel - Error 1`] = `[SyntaxError: Accessibility modifier already seen. (2:9)]`;
diff --git a/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/3-Alignment-Error.shot b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/3-Alignment-Error.shot
new file mode 100644
index 000000000000..d87e3076b189
--- /dev/null
+++ b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/3-Alignment-Error.shot
@@ -0,0 +1,3 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`AST Fixtures element MethodDefinition _error_ duplicated-accessibility-modifiers Error Alignment 1`] = `"Both errored"`;
diff --git a/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/mixed-accessibility-modifiers/fixture.ts b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/mixed-accessibility-modifiers/fixture.ts
new file mode 100644
index 000000000000..6d9a757a6c96
--- /dev/null
+++ b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/mixed-accessibility-modifiers/fixture.ts
@@ -0,0 +1,3 @@
+class Foo {
+  public protected bar() {};
+}
diff --git a/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/1-TSESTree-Error.shot b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/1-TSESTree-Error.shot
new file mode 100644
index 000000000000..adeb1e6df792
--- /dev/null
+++ b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/1-TSESTree-Error.shot
@@ -0,0 +1,10 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`AST Fixtures element MethodDefinition _error_ mixed-accessibility-modifiers TSESTree - Error 1`] = `
+"TSError
+  1 | class Foo {
+> 2 |   public protected bar() {};
+    |          ^^^^^^^^^ Accessibility modifier already seen.
+  3 | }
+  4 |"
+`;
diff --git a/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/2-Babel-Error.shot b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/2-Babel-Error.shot
new file mode 100644
index 000000000000..223e5193276e
--- /dev/null
+++ b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/2-Babel-Error.shot
@@ -0,0 +1,3 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`AST Fixtures element MethodDefinition _error_ mixed-accessibility-modifiers Babel - Error 1`] = `[SyntaxError: Accessibility modifier already seen. (2:9)]`;
diff --git a/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/3-Alignment-Error.shot b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/3-Alignment-Error.shot
new file mode 100644
index 000000000000..957afbc76959
--- /dev/null
+++ b/packages/ast-spec/src/element/MethodDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/3-Alignment-Error.shot
@@ -0,0 +1,3 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`AST Fixtures element MethodDefinition _error_ mixed-accessibility-modifiers Error Alignment 1`] = `"Both errored"`;
diff --git a/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/duplicated-accessibility-modifiers/fixture.ts b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/duplicated-accessibility-modifiers/fixture.ts
new file mode 100644
index 000000000000..cfe758d6b422
--- /dev/null
+++ b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/duplicated-accessibility-modifiers/fixture.ts
@@ -0,0 +1,3 @@
+class Foo {
+  public public bar;
+}
diff --git a/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/1-TSESTree-Error.shot b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/1-TSESTree-Error.shot
new file mode 100644
index 000000000000..6fbf0b6b8290
--- /dev/null
+++ b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/1-TSESTree-Error.shot
@@ -0,0 +1,10 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`AST Fixtures element PropertyDefinition _error_ duplicated-accessibility-modifiers TSESTree - Error 1`] = `
+"TSError
+  1 | class Foo {
+> 2 |   public public bar;
+    |          ^^^^^^ Accessibility modifier already seen.
+  3 | }
+  4 |"
+`;
diff --git a/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/2-Babel-Error.shot b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/2-Babel-Error.shot
new file mode 100644
index 000000000000..1285ca65797e
--- /dev/null
+++ b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/2-Babel-Error.shot
@@ -0,0 +1,3 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`AST Fixtures element PropertyDefinition _error_ duplicated-accessibility-modifiers Babel - Error 1`] = `[SyntaxError: Accessibility modifier already seen. (2:9)]`;
diff --git a/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/3-Alignment-Error.shot b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/3-Alignment-Error.shot
new file mode 100644
index 000000000000..116d47b88301
--- /dev/null
+++ b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/duplicated-accessibility-modifiers/snapshots/3-Alignment-Error.shot
@@ -0,0 +1,3 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`AST Fixtures element PropertyDefinition _error_ duplicated-accessibility-modifiers Error Alignment 1`] = `"Both errored"`;
diff --git a/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/mixed-accessibility-modifiers/fixture.ts b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/mixed-accessibility-modifiers/fixture.ts
new file mode 100644
index 000000000000..a9715175b5ad
--- /dev/null
+++ b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/mixed-accessibility-modifiers/fixture.ts
@@ -0,0 +1,3 @@
+class Foo {
+  public protected bar;
+}
diff --git a/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/1-TSESTree-Error.shot b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/1-TSESTree-Error.shot
new file mode 100644
index 000000000000..0786548487bd
--- /dev/null
+++ b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/1-TSESTree-Error.shot
@@ -0,0 +1,10 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`AST Fixtures element PropertyDefinition _error_ mixed-accessibility-modifiers TSESTree - Error 1`] = `
+"TSError
+  1 | class Foo {
+> 2 |   public protected bar;
+    |          ^^^^^^^^^ Accessibility modifier already seen.
+  3 | }
+  4 |"
+`;
diff --git a/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/2-Babel-Error.shot b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/2-Babel-Error.shot
new file mode 100644
index 000000000000..309038c6d169
--- /dev/null
+++ b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/2-Babel-Error.shot
@@ -0,0 +1,3 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`AST Fixtures element PropertyDefinition _error_ mixed-accessibility-modifiers Babel - Error 1`] = `[SyntaxError: Accessibility modifier already seen. (2:9)]`;
diff --git a/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/3-Alignment-Error.shot b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/3-Alignment-Error.shot
new file mode 100644
index 000000000000..611da1f7d9ca
--- /dev/null
+++ b/packages/ast-spec/src/element/PropertyDefinition/fixtures/_error_/mixed-accessibility-modifiers/snapshots/3-Alignment-Error.shot
@@ -0,0 +1,3 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`AST Fixtures element PropertyDefinition _error_ mixed-accessibility-modifiers Error Alignment 1`] = `"Both errored"`;
diff --git a/packages/typescript-estree/src/convert.ts b/packages/typescript-estree/src/convert.ts
index 2fb094c1b589..3eb898abc8f7 100644
--- a/packages/typescript-estree/src/convert.ts
+++ b/packages/typescript-estree/src/convert.ts
@@ -3419,6 +3419,27 @@ export class Converter {
         );
       }
 
+      // `checkGrammarModifiers` function in `typescript`
+      if (
+        modifier.kind === SyntaxKind.PublicKeyword ||
+        modifier.kind === SyntaxKind.ProtectedKeyword ||
+        modifier.kind === SyntaxKind.PrivateKeyword
+      ) {
+        for (const anotherModifier of getModifiers(node) ?? []) {
+          if (
+            anotherModifier !== modifier &&
+            (anotherModifier.kind === SyntaxKind.PublicKeyword ||
+              anotherModifier.kind === SyntaxKind.ProtectedKeyword ||
+              anotherModifier.kind === SyntaxKind.PrivateKeyword)
+          ) {
+            this.#throwError(
+              anotherModifier,
+              `Accessibility modifier already seen.`,
+            );
+          }
+        }
+      }
+
       // `checkParameter` function in `typescript`
       if (
         node.kind === SyntaxKind.Parameter &&