Handle duplicate keys in connection string

src/EventStore.Client/EventStoreClientSettings.ConnectionString.cs

@@ -220,7 +220,11 @@ private static Dictionary<string, string> ParseKeyValuePairs(string s) {
 				var optionsTokens = s.Split(Ampersand);
 				foreach (var optionToken in optionsTokens) {
 					var (key, val) = ParseKeyValuePair(optionToken);
-					options[key] = val;
+					try {
+						options.Add(key, val);
+					} catch (ArgumentException) {
+						throw new DuplicateKeyException(key);
+					}
 				}
 				return options;
 			}

src/EventStore.Client/Exceptions/ConnectionString/DuplicateKeyException.cs

@@ -0,0 +1,6 @@
+namespace EventStore.Client {
+	public class DuplicateKeyException : ConnectionStringParseException {
+		public DuplicateKeyException(string key)
+			: base($"Duplicate key: '{key}'") { }
+	}
+}

test/EventStore.Client.Tests/ConnectionStringTests.cs

@@ -86,6 +86,17 @@ public void connection_string_with_invalid_key_value_pair_should_throw() {
 			});
 		}
 
+		[Fact]
+		public void connection_string_with_duplicate_key_should_throw() {
+			Assert.Throws<DuplicateKeyException>(() => {
+				EventStoreClientSettings.Create("esdb://user:pass@127.0.0.1/?maxDiscoverAttempts=1234&MaxDiscoverAttempts=10");
+			});
+
+			Assert.Throws<DuplicateKeyException>(() => {
+				EventStoreClientSettings.Create("esdb://user:pass@127.0.0.1/?gossipTimeout=10&gossipTimeout=30");
+			});
+		}
+
 		[Fact]
 		public void connection_string_with_invalid_settings_should_throw() {
 			Assert.Throws<InvalidSettingException>(() => {