This issue is likely the result of an error in the instructions on setting up the 'send to indexer' app that eventually gets deployed to all universal forwarders in a Splunk App for Windows Infrastructure deployment.
A forwarder cannot forward things to itself. When you attempt to do this, messages such as the one in this question appear. As part of setup of the "send to indexer" app, you were instructed to create an outputs.conf that directs forwarders to forward to one or more indexers. If you are working on the same machine, as is assumed in these instructions, then by creating the app on the machine, you unintentionally create a 'forwarder loop' if you follow the steps in the next topic, which says to copy the app out of the apps directory and into the deployment apps directory.
Instead, you should move the app out of the apps directory and into the deployment apps directory. Since a deployment server cannot be a client of itself, it will never receive the "send to indexer" app and this forwarding loop can be avoided.
The documentation has been fixed. Apologies for any inconvenience caused.
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