The send to phantom action will extract artifacts from the splunk result for each row present. If you have multi-value fields in the result then it will extract one for each when using the "send to phantom" adaptive response action.
Best practice is to use the event forwarding portion of the Phantom app, monitoring
notable and configuring the mapping of cef fields to control which fields are sent to Phantom. This also doesn't create multiple artifacts, you will have one with a string of all the multi-values joined together (which could require playbook actions to separate again)
A good way to debug would be to disable all the event forwarders and turn them on one at a time to see which input (or inputs) are causing this.
Also, share the search being used to generate the Splunk events that get sent.