client-v1 (clickhouse-data): "Duplicate key" IllegalStateException on result sets with duplicate column names (0.9.8)

Start in ClickHouseDataProcessor$DefaultSerDe at the constructor location reported in the stack trace, and reproduce with RowBinaryWithNamesAndTypes using the provided duplicate-column query. Check how the column-name-to-index map handles repeated names and verify that positional reads complete successfully. Done means duplicate output names no longer prevent response construction or position-based access.

Description

The v1 client (clickhouse-client / clickhouse-http-client 0.9.8) throws java.lang.IllegalStateException: Duplicate key ... when a query returns a result set with duplicate column names (which is valid SQL).

com.clickhouse.data.ClickHouseDataProcessor$DefaultSerDe builds a column-name → index map with Collectors.toMap (no merge function) at construction time, so any repeated column name fails immediately — before the caller can read anything, even when reading purely by position.

This reproduces with RowBinaryWithNamesAndTypes (the format that carries column names). A trivial SELECT key1, key1 FROM (SELECT 1 AS key1) is enough. Real-world trigger: a reporting tool that lets a user select the same projection twice (e.g. two Date columns), producing SQL like SELECT toDate(...) AS Date, toDate(...) AS Date, ....

This looks related to #2336 (reported on 0.8.4, marked fixed in 0.9.3 via #2618), but that fix was in the JDBC TableSchema (Guava ImmutableMap, "Multiple entries with same key"). The failure here is a different code path — ClickHouseDataProcessor using the JDK Collectors.toMap ("attempted merging values") — and still fails in 0.9.8.

Steps to reproduce

1. Use com.clickhouse:clickhouse-http-client:0.9.8 (v1 client).
2. Execute a query whose result has two columns with the same name, e.g. SELECT key1, key1 FROM (SELECT 1 AS key1), with format RowBinaryWithNamesAndTypes.
3. executeAndWait() / iterating response.records() throws.

Error Log or Exception StackTrace

java.lang.IllegalStateException: Duplicate key key1 (attempted merging values 0 and 1)
    at java.base/java.util.stream.Collectors.duplicateKeyException(Collectors.java:135)
    at java.base/java.util.stream.Collectors.lambda$uniqKeysMapAccumulator$0(Collectors.java:182)
    at java.base/java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
    at java.base/java.util.stream.IntPipeline$1$1.accept(IntPipeline.java:180)
    at java.base/java.util.stream.Streams$RangeIntSpliterator.forEachRemaining(Streams.java:104)
    at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:570)
    at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:560)
    at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:921)
    at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:265)
    at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:727)
    at com.clickhouse.data.ClickHouseDataProcessor$DefaultSerDe.<init>(ClickHouseDataProcessor.java:72)
    at com.clickhouse.client.http.ApacheHttpConnectionImpl ...
    at com.clickhouse.client.ClickHouseClient.executeAndWait(ClickHouseClient.java:894)

Expected Behaviour

Duplicate column names are valid in ClickHouse/SQL, so building the response must not throw. Reading values by position should always work; name-based lookup of a duplicated name can reasonably return the first match (or be documented). Suggestion: build the name→index map with a merge function (keep the first index) instead of the throwing Collectors.toMap.

Code Example

import com.clickhouse.client.*;
import com.clickhouse.data.ClickHouseFormat;
import com.clickhouse.data.ClickHouseRecord;

public class DuplicateColumnRepro {
  public static void main(String[] args) throws Exception {
    ClickHouseNode server = ClickHouseNode.builder()
        .host("localhost")
        .port(ClickHouseProtocol.HTTP, 8123)
        .build();

    try (ClickHouseClient client = ClickHouseClient.newInstance(server.getProtocol());
         ClickHouseResponse response = client.read(server)
             .format(ClickHouseFormat.RowBinaryWithNamesAndTypes)
             .query("SELECT key1, key1 FROM (SELECT 1 AS key1)") // duplicate output column name
             .executeAndWait()) {
      for (ClickHouseRecord r : response.records()) {
        System.out.println(r.getValue(0).asString() + ", " + r.getValue(1).asString());
      }
    }
    // throws: java.lang.IllegalStateException: Duplicate key key1 (attempted merging values 0 and 1)
  }
}

v2 — works (returns 1, 1):

Configuration

Client Configuration

client.read(server)
      .format(ClickHouseFormat.RowBinaryWithNamesAndTypes)
// Reproduces with the default provider; also confirmed with both
// HttpConnectionProvider.APACHE_HTTP_CLIENT and HttpConnectionProvider.HTTP_URL_CONNECTION.

Environment

• Cloud
• Client version: 0.9.8 (clickhouse-http-client / clickhouse-client / clickhouse-data)
• Language version: Java 24.0.2 (also reproduces on Java 17)
• OS: macOS (Darwin arm64); also Linux

ClickHouse Server

• ClickHouse Server version: 26.3.10.60
• ClickHouse Server non-default settings, if any: none relevant
• CREATE TABLE statements for tables involved: none needed — SELECT key1, key1 FROM (SELECT 1 AS key1) reproduces it without any table
• Sample data for all these tables: n/a