Join keys must be used only once found duplicates – How to solve this Elasticsearch exception

Opster Team

August-23, Version: 8.6-8.9

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Briefly, this error occurs when there are duplicate join keys in an Elasticsearch query. Join keys are used to link documents in a parent-child relationship. Having duplicate join keys can cause confusion and incorrect results. To resolve this issue, ensure that each join key is unique. You can do this by checking your data before indexing or by using a script to automatically generate unique join keys. Also, review your queries to ensure they are not creating duplicate join keys.

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This guide will help you check for common problems that cause the log ” Join keys must be used only once; found duplicates: [{}] ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: parser, plugin, join.

Log Context

Log “Join keys must be used only once; found duplicates: [{}]” class name is We extracted the following from Elasticsearch source code for those seeking an in-depth context :

 Source src = source( != null ? : joinTermCtx);
 StringJoiner duplicates = new StringJoiner(";");
 for (String duplicate : duplicateKeyNames) {
 throw new ParsingException(src; "Join keys must be used only once; found duplicates: [{}]"; duplicates.toString());
 }  if (queries.size() < 2) {
 throw new ParsingException(source; "A sample requires a minimum of 2 queries; found [{}]"; queries.size());


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