Fix duplicate keys causing an exception

Jellyfin.Server/Middleware/UrlDecodeQueryFeature.cs

@@ -68,15 +68,20 @@ namespace Jellyfin.Server.Middleware
                 foreach (var pair in queryString)
                 {
                     var i = pair.IndexOf('=');
-
                     if (i == -1)
                     {
                         // encoded is an equals.
-                        pairs.Add(pair.ToString(), StringValues.Empty);
+                        // We use TryAdd so duplicate keys get ignored
+                        pairs.TryAdd(pair.ToString(), StringValues.Empty);
                         continue;
                     }
 
-                    pairs.Add(pair[..i].ToString(), new StringValues(pair[(i + 1)..].ToString()));
+                    var k = pair[..i].ToString();
+                    var v = pair[(i + 1)..].ToString();
+                    if (!pairs.TryAdd(k, new StringValues(v)))
+                    {
+                        pairs[k] = StringValues.Concat(pairs[k], v);
+                    }
                 }
 
                 _store = new QueryCollection(pairs);

tests/Jellyfin.Server.Integration.Tests/Controllers/EncoderController.cs

@@ -29,5 +29,25 @@ namespace Jellyfin.Api.Controllers
                 StatusCode = 200
             };
         }
+
+        /// <summary>
+        /// Tests the url decoding.
+        /// </summary>
+        /// <param name="params">Parameters to echo back in the response.</param>
+        /// <returns>An <see cref="OkResult"/>.</returns>
+        /// <response code="200">Information retrieved.</response>
+        [HttpGet("UrlArrayDecode")]
+        [ProducesResponseType(StatusCodes.Status200OK)]
+        public ContentResult TestUrlArrayDecoding([FromQuery] Dictionary<string, string[]>? @params = null)
+        {
+            return new ContentResult()
+            {
+                Content = (@params != null && @params.Count > 0)
+                    ? string.Join("&", @params.Select(x => x.Key + "=" + string.Join(',', x.Value)))
+                    : string.Empty,
+                ContentType = "text/plain; charset=utf-8",
+                StatusCode = 200
+            };
+        }
     }
 }

tests/Jellyfin.Server.Integration.Tests/EncodedQueryStringTest.cs

@@ -20,6 +20,8 @@ namespace Jellyfin.Server.Integration.Tests
         [InlineData("a=1&b=2&c=3", "a=1&b=2&c=3")] // won't be processed as there is more than 1.
         [InlineData("a=1", "a=1")] // won't be processed as it has a value
         [InlineData("a%3D1%26b%3D2%26c%3D3", "a=1&b=2&c=3")] // will be processed.
+        [InlineData("a=b&a=c", "a=b")]
+        [InlineData("a%3Db%26a%3Dc", "a=b")]
         public async Task Ensure_Decoding_Of_Urls_Is_Working(string sourceUrl, string unencodedUrl)
         {
             var client = _factory.CreateClient();
@@ -29,5 +31,18 @@ namespace Jellyfin.Server.Integration.Tests
             string reply = await response.Content.ReadAsStringAsync().ConfigureAwait(false);
             Assert.Equal(unencodedUrl, reply);
         }
+
+        [Theory]
+        [InlineData("a=b&a=c", "a=b,c")]
+        [InlineData("a%3Db%26a%3Dc", "a=b,c")]
+        public async Task Ensure_Array_Decoding_Of_Urls_Is_Working(string sourceUrl, string unencodedUrl)
+        {
+            var client = _factory.CreateClient();
+
+            var response = await client.GetAsync("Encoder/UrlArrayDecode?" + sourceUrl).ConfigureAwait(false);
+            Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+            string reply = await response.Content.ReadAsStringAsync().ConfigureAwait(false);
+            Assert.Equal(unencodedUrl, reply);
+        }
     }
 }

tests/Jellyfin.Server.Tests/UrlDecodeQueryFeatureTests.cs

@@ -14,6 +14,7 @@ namespace Jellyfin.Server.Tests
         [InlineData("e0a72cb2a2c7", "e0a72cb2a2c7")] // isn't encoded
         [InlineData("random+test", "random test")] // encoded
         [InlineData("random%20test", "random test")] // encoded
+        [InlineData("++", "  ")] // encoded
         public static void EmptyValueTest(string query, string key)
         {
             var dict = new Dictionary<string, StringValues>
@@ -23,7 +24,7 @@ namespace Jellyfin.Server.Tests
             var test = new UrlDecodeQueryFeature(new QueryFeature(new QueryCollection(dict)));
             Assert.Single(test.Query);
             var (k, v) = test.Query.First();
-            Assert.Equal(k, key);
+            Assert.Equal(key, k);
             Assert.Empty(v);
         }
     }