Deal with duplicate variables

```r
# A tibble: 8 x 4
Variable Code Units EC_Code

1 Chromium Total EMS_CR_T ug/L 463
2 Chromium Total EMS_CR_T mg/L 463
3 Conductivity Specific EMS_SC_F mS/cm NA
4 Conductivity Specific EMS_0011 mS/cm NA
5 Temperature EMS_0013 degC 840
6 Temperature EMS_TEMF degC NA
7 Turbidity EMS_TURF NTU NA
8 Turbidity EMS_0015 NTU 864
```

In particular set Chromium to have the units in the limits table.

Distinguish between Temperature and Temperature - Field (TEMF)
Treat Conductivity and Turbidity as the same with the same EC_Code