Timestamp  Status
1501       Normal
1501       Normal
1502       Delay
1503       Received
1504       Normal
1504       Delay
1505       Received
1506       Received
1507       Delay
1507       Received

Timestamp  Status     Notif
1501       Normal     N0
1501       Normal     N0
1502       Delay      N0
1503       Received   N1
1504       Normal     N1
1504       Delay      N1
1505       Received   N2
1506       Received   N3
1507       Delay      N3
1507       Received   N4

Timestamp  Status     Notif
1501       Normal     N0
1501       Normal
1502       Delay
1503       Received   N1
1504       Normal
1504       Delay
1505       Received   N2
1506       Received   N3
1507       Delay
1507       Received   N4

df['Notif'] = None
counter = 0
for idx, row in df.iterrows():
    if df.iloc[idx, 1] == "Received":
        counter +=1
    df.iloc[idx,-1] = "N" + str(counter)

df.drop_duplicates(subset='Notif', keep="first")