icontrol - import policy create new policy instead overwriting of old one

Try this (untested):

{
  "fileName": "sp_test_api.json",
  "policy":{
    "fullPath": "/Common/sp_test_api_baseline",
   }
}

I personally do not upload the policy, but use file references:

{
  "fileReference":{
    "link": http://webserver/policy.json
  },
  "policy":{
    "fullPath": "/Common/sp_test_api_baseline",
  }
}