How to append a list selectively?

I'm receiving my messages via a RFID reader. What I am trying to do is to remove duplicates and only append to a list following two conditions:

1. If their ID (epc in this case) is unique and, (done)

2. The datetime is after a 5 minutes interval (so I can keep track of the same tag is still being read by the RFID reader every 5 mins)

   import paho.mqtt.client as mqtt
   import json
   
   testlist = []       
   
   def on_message(client, userdata, msg):
       payloadjson = json.loads(msg.payload.decode('utf-8'))
       line = payloadjson["value"].split(',')
       epc =  line[1]
       datetime = payloadjson['datetime']
       # datetime is in this string format '2016-04-06 03:21:17'
   
       payload = {'datetime': datetime, 'epc': epc[11:35]}
   
       # this if-statement satisfy condition 1
       if payload not in testlist:
           testlist.append(payload)
           for each in teslist:
               print (each)
   
   

How can I achieve condition 2?