Correctly handle claims of same type with multiple values

src/Microsoft.IdentityModel.Tokens/TokenUtilities.cs

@@ -128,13 +128,17 @@ internal static IDictionary<string, object> CreateDictionaryFromClaims(
                 // When the creating the JWT and a list is found, a JsonArray will be created.
                 if (payload.TryGetValue(claim.Type, out object existingValue))
                 {
-                    if (existingValue is not IList<object>)
+                    if (existingValue is IList<object> existingList)
+                    {
+                        existingList.Add(jsonClaimValue);
+                    }
+                    else
                     {
                         payload[claim.Type] = new List<object>
                         {
                             existingValue,
                             jsonClaimValue
-                        }; 
+                        };
                     }
                 }
                 else

test/Microsoft.IdentityModel.JsonWebTokens.Tests/JsonWebTokenHandlerTests.cs

@@ -28,6 +28,40 @@ namespace Microsoft.IdentityModel.JsonWebTokens.Tests
 {
     public class JsonWebTokenHandlerTests
     {
+        [Fact]
+        public void JsonWebTokenHandler_CreateToken_SameTypeMultipleValues()
+        {
+            var identity = new ClaimsIdentity("Test");
+
+            var claimValues = new List<string> { "value1", "value2", "value3", "value4" };
+
+            foreach (var value in claimValues)
+                identity.AddClaim(new Claim("a", value));
+
+            var descriptor = new SecurityTokenDescriptor
+            {
+                SigningCredentials = new SigningCredentials(Default.AsymmetricSigningKey, SecurityAlgorithms.RsaSsaPssSha256),
+                Subject = identity
+            };
+
+            var handler = new JsonWebTokenHandler();
+
+            var token = handler.CreateToken(descriptor);
+
+            var jwt = new JsonWebToken(token);
+            var claims = jwt.Claims.ToList();
+
+            int defaultClaimsCount = 3;
+
+            Assert.Equal(defaultClaimsCount + claimValues.Count, claims.Count);
+
+            var aTypeClaims = claims.Where(c => c.Type == "a").ToList();
+
+            Assert.Equal(4, aTypeClaims.Count);
+
+            foreach (var value in claimValues)
+                Assert.NotNull(aTypeClaims.SingleOrDefault(c => c.Value == value));
+        }
 
         // This test checks to make sure that the value of JsonWebTokenHandler.Base64UrlEncodedUnsignedJWSHeader has remained unchanged.
         [Fact]

test/System.IdentityModel.Tokens.Jwt.Tests/JwtSecurityTokenHandlerTests.cs

@@ -20,6 +20,40 @@ namespace System.IdentityModel.Tokens.Jwt.Tests
     public class JwtSecurityTokenHandlerTests
     {
 
+        [Fact]
+        public void JwtSecurityTokenHandler_CreateToken_SameTypeMultipleValues()
+        {
+            var identity = new ClaimsIdentity("Test");
+
+            var claimValues = new List<string> { "value1", "value2", "value3", "value4" };
+
+            foreach (var value in claimValues)
+                identity.AddClaim(new Claim("a", value));
+
+            var descriptor = new SecurityTokenDescriptor
+            {
+                SigningCredentials = new SigningCredentials(Default.AsymmetricSigningKey, SecurityAlgorithms.RsaSsaPssSha256),
+                Subject = identity
+            };
+
+            var handler = new JwtSecurityTokenHandler();
+
+            var jwt = (JwtSecurityToken)handler.CreateToken(descriptor);
+
+            var claims = jwt.Claims.ToList();
+
+            int defaultClaimsCount = 3;
+
+            Assert.Equal(defaultClaimsCount + claimValues.Count, claims.Count);
+
+            var aTypeClaims = claims.Where(c => c.Type == "a").ToList();
+
+            Assert.Equal(4, aTypeClaims.Count);
+
+            foreach (var value in claimValues)
+                Assert.NotNull(aTypeClaims.SingleOrDefault(c => c.Value == value));
+        }
+
         [Theory, MemberData(nameof(CreateJWEWithPayloadStringTheoryData))]
         public void CreateJWEWithAdditionalHeaderClaims(CreateTokenTheoryData theoryData)
         {