Mxpanel.People.set_once

You're seeing just the function set_once, go back to Mxpanel.People module for more information.
Link to this function

set_once(client, distinct_id, properties, opts \\ [])

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Specs

set_once(Mxpanel.Client.t(), String.t(), map(), Keyword.t()) ::
  :ok | {:error, term()}

Works just like set/4 except it will not overwrite existing property values. This is useful for properties like "First login date".

properties = %{"First login date" => "2013-04-01T13:20:00"}
Mxpanel.People.set_once(client, "13793", properties)