Duplicate values stored in database

I've run into one critical issue and found that influx has duplicate value on a single tag - timestamp pair.

```
> SELECT "time", "2001", "SensorId" FROM dbname.autogen.Reading WHERE "SensorId" = '17186' AND time >= 1557953095198000000 and time <= 1557953103853000000 GROUP BY "SensorId";
name: Reading
tags: SensorId=17186
time 2001 SensorId
---- ---- --------
1557953099000000000 330638950.4 17186
1557953099000000000 330967451.0651455 17186
```

**Influx Version: 1.7.4**

__Actual Behaviour__
Duplicate field values stored for one tag and timestamp pair.

__Expected Behaviour__
The value should have override and only one value should have present.

Start by reproducing the supplied InfluxDB 1.7.4 SELECT query with two writes at the same tag and timestamp, then inspect the write and storage paths involved in series-field updates. Done means identifying why both field values remain and confirming that the expected overwrite behavior is covered by a regression test.

Written by the indexing model from the issue text.