diff --git a/packages/babylon/src/tokenizer/index.js b/packages/babylon/src/tokenizer/index.js
index ba34eb3f5f..1396bf7d0b 100644
--- a/packages/babylon/src/tokenizer/index.js
+++ b/packages/babylon/src/tokenizer/index.js
@@ -20,6 +20,8 @@ import {
 } from "../util/whitespace";
 import State from "./state";
 
+const VALID_REGEX_FLAGS = "gmsiyu";
+
 // The following character codes are forbidden from being
 // an immediate sibling of NumericLiteralSeparator _
 
@@ -834,19 +836,24 @@ export default class Tokenizer extends LocationParser {
     const content = this.input.slice(start, this.state.pos);
     ++this.state.pos;
 
-    const validFlags = /^[gmsiyu]$/;
     let mods = "";
+
     while (this.state.pos < this.input.length) {
       const char = this.input[this.state.pos];
       const charCode = this.fullCharCodeAtPos();
-      if (validFlags.test(char)) {
+
+      if (VALID_REGEX_FLAGS.indexOf(char) > -1) {
+        if (mods.indexOf(char) > -1) {
+          this.raise(this.state.pos + 1, "Duplicate regular expression flag");
+        }
+
         ++this.state.pos;
         mods += char;
       } else if (
         isIdentifierChar(charCode) ||
         charCode === charCodes.backslash
       ) {
-        this.raise(this.state.pos, "Invalid regular expression flag");
+        this.raise(this.state.pos + 1, "Invalid regular expression flag");
       } else {
         break;
       }
diff --git a/packages/babylon/test/fixtures/core/uncategorised/108/options.json b/packages/babylon/test/fixtures/core/uncategorised/108/options.json
index adf556b0a3..080cb0eaaa 100644
--- a/packages/babylon/test/fixtures/core/uncategorised/108/options.json
+++ b/packages/babylon/test/fixtures/core/uncategorised/108/options.json
@@ -1,3 +1,3 @@
 {
-  "throws": "Invalid regular expression flag (1:16)"
+  "throws": "Invalid regular expression flag (1:17)"
 }
diff --git a/packages/babylon/test/fixtures/core/uncategorised/366/options.json b/packages/babylon/test/fixtures/core/uncategorised/366/options.json
index d67d8472c0..adf556b0a3 100644
--- a/packages/babylon/test/fixtures/core/uncategorised/366/options.json
+++ b/packages/babylon/test/fixtures/core/uncategorised/366/options.json
@@ -1,3 +1,3 @@
 {
-  "throws": "Invalid regular expression flag (1:15)"
+  "throws": "Invalid regular expression flag (1:16)"
 }
diff --git a/packages/babylon/test/fixtures/es2015/regex/duplicate-flags/input.js b/packages/babylon/test/fixtures/es2015/regex/duplicate-flags/input.js
new file mode 100644
index 0000000000..1c21dcd778
--- /dev/null
+++ b/packages/babylon/test/fixtures/es2015/regex/duplicate-flags/input.js
@@ -0,0 +1 @@
+/./gii;
diff --git a/packages/babylon/test/fixtures/es2015/regex/duplicate-flags/options.json b/packages/babylon/test/fixtures/es2015/regex/duplicate-flags/options.json
new file mode 100644
index 0000000000..86670c5168
--- /dev/null
+++ b/packages/babylon/test/fixtures/es2015/regex/duplicate-flags/options.json
@@ -0,0 +1,3 @@
+{
+  "throws": "Duplicate regular expression flag (1:6)"
+}
diff --git a/scripts/tests/test262/test262_whitelist.txt b/scripts/tests/test262/test262_whitelist.txt
index e10be4e105..3a65e6e492 100644
--- a/scripts/tests/test262/test262_whitelist.txt
+++ b/scripts/tests/test262/test262_whitelist.txt
@@ -1946,8 +1946,6 @@ language/global-code/new.target-arrow.js(default)
 language/global-code/new.target-arrow.js(strict mode)
 language/import/dup-bound-names.js(default)
 language/import/dup-bound-names.js(strict mode)
-language/literals/regexp/early-err-dup-flag.js(default)
-language/literals/regexp/early-err-dup-flag.js(strict mode)
 language/literals/regexp/early-err-pattern.js(default)
 language/literals/regexp/early-err-pattern.js(strict mode)
 language/literals/regexp/invalid-braced-quantifier-exact.js(default)