Date    Value
0   2017-07-18 07:40:00 1
1   2017-07-18 07:45:00 1
2   2017-07-18 07:50:00 1
3   2017-07-18 07:55:00 2414
4   2017-07-18 08:00:00 2
5   2017-07-18 08:05:00 2
6   2017-07-18 08:10:00 4416
7   2017-07-18 08:15:00 4416
8   2017-07-18 08:20:00 3
9   2017-07-18 08:25:00 3
10  2017-07-18 08:30:00 3
11  2017-07-18 08:35:00 6998

df['Value'] = df['Value'].loc[df['Value'].shift() != df['Value']]

Date    Value
0   2017-07-18 07:40:00 1.0
1   2017-07-18 07:45:00 NaN
2   2017-07-18 07:50:00 NaN
3   2017-07-18 07:55:00 2414.0
4   2017-07-18 08:00:00 2.0
5   2017-07-18 08:05:00 NaN
6   2017-07-18 08:10:00 4416.0
7   2017-07-18 08:15:00 NaN
8   2017-07-18 08:20:00 3.0
9   2017-07-18 08:25:00 NaN
10  2017-07-18 08:30:00 NaN
11  2017-07-18 08:35:00 6998.0

Date    Value
0   2017-07-18 07:40:00 NaN
1   2017-07-18 07:45:00 NaN
2   2017-07-18 07:50:00 NaN
3   2017-07-18 07:55:00 2414.0
4   2017-07-18 08:00:00 2.0
5   2017-07-18 08:05:00 2.0
6   2017-07-18 08:10:00 4416.0
7   2017-07-18 08:15:00 4416.0
8   2017-07-18 08:20:00 NaN
9   2017-07-18 08:25:00 NaN
10  2017-07-18 08:30:00 NaN
11  2017-07-18 08:35:00 6998.0