Sitecore Solr - multiple values encountered for non multiValued field

In case if you have faced an error "multiple values encountered for non multiValued field" in Solr, you will need to find the fieldType element in the Sitecore Solr Index Configuration and set it as stringCollection in returnType.

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <contentSearch>
      <indexConfigurations>
        <defaultSolrIndexConfiguration type="Sitecore.ContentSearch.SolrProvider.SolrIndexConfiguration, Sitecore.ContentSearch.SolrProvider">
          <fieldMap ref="contentSearch/indexConfigurations/defaultSolrIndexConfiguration/fieldMap">
            <fieldNames hint="raw:AddFieldByFieldName">
              <fieldType fieldName="tags" returnType="stringCollection"/>
            </fieldNames>
          </fieldMap>
        </defaultSolrIndexConfiguration>
      </indexConfigurations>
    </contentSearch>
  </sitecore>
</configuration>

Error:

org.apache.solr.common.SolrException: ERROR: [doc=sitecore://master/{37ac1cc6-109b-41ee-9f94-3f527ff22c12}?lang=en&ver=1&ndx=sitecore_master_index] multiple values encountered for non multiValued field tags_t: [594396e563f046b98418b3a8a52d4aa2, 5e918d1e4412450298a8eae0ee65da65, 9fe55245aa6641b79ae55cd47874ee0e]
	at org.apache.solr.update.DocumentBuilder.toDocument(DocumentBuilder.java:153)
	at org.apache.solr.update.AddUpdateCommand.getLuceneDocument(AddUpdateCommand.java:109)
	at org.apache.solr.update.DirectUpdateHandler2.updateDocOrDocValues(DirectUpdateHandler2.java:975)
	at org.apache.solr.update.DirectUpdateHandler2.doNormalUpdate(DirectUpdateHandler2.java:345)
	at org.apache.solr.update.DirectUpdateHandler2.addDoc0(DirectUpdateHandler2.java:292)
	at org.apache.solr.update.DirectUpdateHandler2.addDoc(DirectUpdateHandler2.java:239)
	at org.apache.solr.update.processor.RunUpdateProcessor.processAdd(RunUpdateProcessorFactory.java:76)
	at org.apache.solr.update.processor.UpdateRequestProcessor.processAdd(UpdateRequestProcessor.java:55)
	at org.apache.solr.update.processor.DistributedUpdateProcessor.doLocalAdd(DistributedUpdateProcessor.java:259)
	at org.apache.solr.update.processor.DistributedUpdateProcessor.doVersionAdd(DistributedUpdateProcessor.java:489)
	at org.apache.solr.update.processor.DistributedUpdateProcessor.lambda$versionAdd$0(DistributedUpdateProcessor.java:339)
	at org.apache.solr.update.VersionBucket.runWithLock(VersionBucket.java:50)
	at org.apache.solr.update.processor.DistributedUpdateProcessor.versionAdd(DistributedUpdateProcessor.java:339)
	at org.apache.solr.update.processor.DistributedUpdateProcessor.processAdd(DistributedUpdateProcessor.java:225)
	at org.apache.solr.update.processor.LogUpdateProcessorFactory$LogUpdateProcessor.processAdd(LogUpdateProcessorFactory.java:103)
	at org.apache.solr.handler.loader.XMLLoader.processUpdate(XMLLoader.java:261)
	at org.apache.solr.handler.loader.XMLLoader.load(XMLLoader.java:188)
	at org.apache.solr.handler.UpdateRequestHandler$1.load(UpdateRequestHandler.java:97)
	at org.apache.solr.handler.ContentStreamHandlerBase.handleRequestBody(ContentStreamHandlerBase.java:68)
	at org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:211)
	at org.apache.solr.core.SolrCore.execute(SolrCore.java:2596)
	at org.apache.solr.servlet.HttpSolrCall.execute(HttpSolrCall.java:799)
	at org.apache.solr.servlet.HttpSolrCall.call(HttpSolrCall.java:578)
	at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:419)
	at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:351)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1602)
	at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:540)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:146)
	at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:548)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:257)
	at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1711)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:255)
	at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1347)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:203)
	at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:480)
	at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1678)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:201)
	at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1249)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:144)
	at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:220)
	at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:152)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
	at org.eclipse.jetty.rewrite.handler.RewriteHandler.handle(RewriteHandler.java:335)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
	at org.eclipse.jetty.server.Server.handle(Server.java:505)
	at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:370)
	at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:267)
	at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:305)
	at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:103)
	at org.eclipse.jetty.io.ssl.SslConnection$DecryptedEndPoint.onFillable(SslConnection.java:427)
	at org.eclipse.jetty.io.ssl.SslConnection.onFillable(SslConnection.java:321)
	at org.eclipse.jetty.io.ssl.SslConnection$2.succeeded(SslConnection.java:159)
	at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:103)
	at org.eclipse.jetty.io.ChannelEndPoint$2.run(ChannelEndPoint.java:117)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.runTask(EatWhatYouKill.java:333)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:310)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.tryProduce(EatWhatYouKill.java:168)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:126)
	at org.eclipse.jetty.util.thread.ReservedThreadExecutor$ReservedThread.run(ReservedThreadExecutor.java:366)
	at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:781)
	at org.eclipse.jetty.util.thread.QueuedThreadPool$Runner.run(QueuedThreadPool.java:917)
	at java.base/java.lang.Thread.run(Thread.java:829)