diff --git a/src/Microsoft.AspNetCore.Razor.Language/Legacy/CSharpCodeParser.cs b/src/Microsoft.AspNetCore.Razor.Language/Legacy/CSharpCodeParser.cs
index bbefbd01c2..2b5eceda1e 100644
--- a/src/Microsoft.AspNetCore.Razor.Language/Legacy/CSharpCodeParser.cs
+++ b/src/Microsoft.AspNetCore.Razor.Language/Legacy/CSharpCodeParser.cs
@@ -1877,9 +1877,28 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
 
         protected virtual void TagHelperPrefixDirective()
         {
+            RazorDiagnostic duplicateDiagnostic = null;
+            if (Context.SeenDirectives.Contains(SyntaxConstants.CSharp.TagHelperPrefixKeyword))
+            {
+                // There wil always be at least 1 child because of the `@` transition.
+                var directiveStart = Context.Builder.CurrentBlock.Children.First().Start;
+                var errorLength = /* @ */ 1 + SyntaxConstants.CSharp.TagHelperPrefixKeyword.Length;
+                duplicateDiagnostic = RazorDiagnosticFactory.CreateParsing_DuplicateDirective(
+                    SyntaxConstants.CSharp.TagHelperPrefixKeyword,
+                    new SourceSpan(directiveStart, errorLength));
+            }
+
             TagHelperDirective(
                 SyntaxConstants.CSharp.TagHelperPrefixKeyword,
-                (prefix, errors) => new TagHelperPrefixDirectiveChunkGenerator(prefix, errors));
+                (prefix, errors) =>
+                {
+                    if (duplicateDiagnostic != null)
+                    {
+                        errors.Add(duplicateDiagnostic);
+                    }
+
+                    return new TagHelperPrefixDirectiveChunkGenerator(prefix, errors);
+                });
         }
 
         protected virtual void AddTagHelperDirective()
diff --git a/src/Microsoft.AspNetCore.Razor.Language/RazorDiagnosticFactory.cs b/src/Microsoft.AspNetCore.Razor.Language/RazorDiagnosticFactory.cs
index 52d7bb2997..80e5d6043c 100644
--- a/src/Microsoft.AspNetCore.Razor.Language/RazorDiagnosticFactory.cs
+++ b/src/Microsoft.AspNetCore.Razor.Language/RazorDiagnosticFactory.cs
@@ -45,6 +45,16 @@ namespace Microsoft.AspNetCore.Razor.Language
             return RazorDiagnostic.Create(CodeTarget_UnsupportedExtension, SourceSpan.Undefined, documentKind, extensionType.Name);
         }
 
+        internal static readonly RazorDiagnosticDescriptor Parsing_DuplicateDirective =
+            new RazorDiagnosticDescriptor(
+                $"{DiagnosticPrefix}2001",
+                () => Resources.DuplicateDirective,
+                RazorDiagnosticSeverity.Error);
+        public static RazorDiagnostic CreateParsing_DuplicateDirective(string directive, SourceSpan location)
+        {
+            return RazorDiagnostic.Create(Parsing_DuplicateDirective, location, directive);
+        }
+
         #endregion
 
         #region TagHelper Errors
diff --git a/test/Microsoft.AspNetCore.Razor.Language.Test/Legacy/CSharpDirectivesTest.cs b/test/Microsoft.AspNetCore.Razor.Language.Test/Legacy/CSharpDirectivesTest.cs
index 1c1c9aa6a1..dc3da3bb61 100644
--- a/test/Microsoft.AspNetCore.Razor.Language.Test/Legacy/CSharpDirectivesTest.cs
+++ b/test/Microsoft.AspNetCore.Razor.Language.Test/Legacy/CSharpDirectivesTest.cs
@@ -1113,6 +1113,29 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
                         .Accepts(AcceptedCharactersInternal.None)));
         }
 
+        [Fact]
+        public void TagHelperPrefixDirective_DuplicatesCauseError()
+        {
+            // Arrange
+            var expectedDiagnostic = RazorDiagnosticFactory.CreateParsing_DuplicateDirective(
+                "tagHelperPrefix",
+                new SourceSpan(null, 22 + Environment.NewLine.Length, 1, 0, 16));
+
+            // Act
+            var document = ParseDocument(
+@"@tagHelperPrefix ""th:""
+@tagHelperPrefix ""th""",
+                directives: null,
+                designTime: false);
+
+            // Assert
+            var directive = document.Root.Children.OfType<Block>().Last();
+            var erroredSpan = (Span)directive.Children.Last();
+            var chunkGenerator = Assert.IsType<TagHelperPrefixDirectiveChunkGenerator>(erroredSpan.ChunkGenerator);
+            var diagnostic = Assert.Single(chunkGenerator.Diagnostics);
+            Assert.Equal(expectedDiagnostic, diagnostic);
+        }
+
         [Fact]
         public void TagHelperPrefixDirective_NoValueSucceeds()
         {
diff --git a/test/Microsoft.AspNetCore.Razor.Language.Test/TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives_DesignTime.diagnostics.txt b/test/Microsoft.AspNetCore.Razor.Language.Test/TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives_DesignTime.diagnostics.txt
index 749ecb184e..193cdac5e8 100644
--- a/test/Microsoft.AspNetCore.Razor.Language.Test/TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives_DesignTime.diagnostics.txt
+++ b/test/Microsoft.AspNetCore.Razor.Language.Test/TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives_DesignTime.diagnostics.txt
@@ -6,7 +6,9 @@ TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cs
 TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(9,18): Error RZ9999: Unterminated string literal.  Strings that start with a quotation mark (") must be terminated before the end of the line.  However, strings that start with @ and a quotation mark (@") can span multiple lines.
 TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(11,2): Error RZ9999: Directive 'tagHelperPrefix' must have a value.
 TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(12,2): Error RZ9999: Directive 'tagHelperPrefix' must have a value.
+TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(12,1): Error RZ2001: The 'tagHelperPrefix' directive may only occur once per document.
 TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(13,18): Error RZ9999: Unterminated string literal.  Strings that start with a quotation mark (") must be terminated before the end of the line.  However, strings that start with @ and a quotation mark (@") can span multiple lines.
+TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(13,1): Error RZ2001: The 'tagHelperPrefix' directive may only occur once per document.
 TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(15,10): Error RZ9999: The 'inherits' directive expects a type name.
 TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(16,1): Error RZ9999: The 'inherits' directive may only occur once per document.
 TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(16,11): Error RZ9999: The 'inherits' directive expects a type name.
diff --git a/test/Microsoft.AspNetCore.Razor.Language.Test/TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives_Runtime.diagnostics.txt b/test/Microsoft.AspNetCore.Razor.Language.Test/TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives_Runtime.diagnostics.txt
index 749ecb184e..193cdac5e8 100644
--- a/test/Microsoft.AspNetCore.Razor.Language.Test/TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives_Runtime.diagnostics.txt
+++ b/test/Microsoft.AspNetCore.Razor.Language.Test/TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives_Runtime.diagnostics.txt
@@ -6,7 +6,9 @@ TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cs
 TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(9,18): Error RZ9999: Unterminated string literal.  Strings that start with a quotation mark (") must be terminated before the end of the line.  However, strings that start with @ and a quotation mark (@") can span multiple lines.
 TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(11,2): Error RZ9999: Directive 'tagHelperPrefix' must have a value.
 TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(12,2): Error RZ9999: Directive 'tagHelperPrefix' must have a value.
+TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(12,1): Error RZ2001: The 'tagHelperPrefix' directive may only occur once per document.
 TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(13,18): Error RZ9999: Unterminated string literal.  Strings that start with a quotation mark (") must be terminated before the end of the line.  However, strings that start with @ and a quotation mark (@") can span multiple lines.
+TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(13,1): Error RZ2001: The 'tagHelperPrefix' directive may only occur once per document.
 TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(15,10): Error RZ9999: The 'inherits' directive expects a type name.
 TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(16,1): Error RZ9999: The 'inherits' directive may only occur once per document.
 TestFiles/IntegrationTests/CodeGenerationIntegrationTest/IncompleteDirectives.cshtml(16,11): Error RZ9999: The 'inherits' directive expects a type name.