How to add non-unique elements to existing array in Firestore?

I am using Firestore to store time series data being pulled from a sensor. I am using Python to push the data, namely the Firebase-Admin package for verification. I chose to store this data using arrays, where each index corresponds with an array across different fields. Is there a way to add non-unique elements to the array? Or can arrays only store unique elements? If so, what data structure would you suggest for storing time series data.

I am trying to add observations to an existing array in Firestore, but ArrayUpdate only adds the element if it is not already present in the array. When I execute the second chuck of code (to update the existing array), only unique values are saved

# Initialize arrays and push to Firestore
import firebase_admin
from firebase_admin import credentials
from firebase_admin import firestore
import datetime

cred = credentials.Certificate('path_to_certificate')

firebase_admin.initialize_app(cred)
db = firestore.client()
cell_1_arr = []
cell_2_arr = []
cell_3_arr = []
exec_time_arr = []
curr_time_arr = []
pred_volt = 12562.70
meas_volt = 12362.70
current = 0.0
soc = 0,0
cell_1_volt = 4.32
cell_2_volt = 4.4
cell_3_volt = 4.23
exec_time = 0.4
curr_time = datetime.datetime.now()
pred_v_arr.append(pred_volt)
meas_v_arr.append(meas_volt)
c_arr.append(current)
soc_arr.append(soc)
cell_1_arr.append(cell_1_volt)
cell_2_arr.append(cell_2_volt)
cell_3_arr.append(cell_3_volt)
exec_time_arr.append(exec_time)
curr_time_arr.append(curr_time)

try:
    push_data = {
        u'time': curr_time_arr,
        u'vpred': meas_v_arr,
        u'vmeas': pred_v_arr,
        u'current': c_arr,
        u'soc': soc_arr,
        u'cell1': cell_1_arr,
        u'cell2': cell_2_arr,
        u'cell3': cell_3_arr,
        u'exectime': exec_time_arr
    }
    db.collection(u'battery1').document(u"day1").set(push_data)

# Add add a new observation to the different arrays
db.collection(u'battery1').document(u"day1").update({'time': firestore.ArrayUnion(curr_time_arr)})
db.collection(u'battery1').document(u"day1").update({'vpred': firestore.ArrayUnion(pred_v_arr)})
db.collection(u'battery1').document(u"day1").update({'vmeas': firestore.ArrayUnion(meas_v_arr)})
db.collection(u'battery1').document(u"day1").update({'current': firestore.ArrayUnion(c_arr)})
db.collection(u'battery1').document(u"day1").update({'cell1': firestore.ArrayUnion(cell_1_arr)})
db.collection(u'battery1').document(u"day1").update({'cell2': firestore.ArrayUnion(cell_2_arr)})
db.collection(u'battery1').document(u"day1").update({'cell3': firestore.ArrayUnion(cell_3_arr)})
db.collection(u'battery1').document(u"day1").update({'exectime': firestore.ArrayUnion(exec_time_arr)})
db.collection(u'battery1').document(u"day1").update({'soc': firestore.ArrayUnion(soc_arr)})

In the screenshot above you can see that there are 8 elements in the "time" field (as all calls to datetime.now() produce unique instances of timestamps), while all the other fields have only saved the unique data points sent (exectime/soc only have two data points, for 8 calls to ArrayUnion).