Bug Report: drop_duplicates inconsistent result
Hi Pandas Community,

```
I found out one potential bug about drop_duplicates producing inconsistent result, with only a switch order of two columns. My Pandas version is 0.17.0. The data set is in Walmart - TripType classification question. https://www.kaggle.com/c/walmart-recruiting-trip-type-classification/data?train.csv.zip

Following is my code: 
```

```
import pandas as pd
df = pd.read_csv('train.csv')
df_meta = df[['VisitNumber', 'TripType', 'Weekday']].drop_duplicates()
print df_meta.shape
```

Return result (95622, 3)

```
import pandas as pd
df = pd.read_csv('train.csv')
df_meta = df[['TripType', 'VisitNumber', 'Weekday']].drop_duplicates()
print df_meta.shape
```

Return result (95674, 3)

The only difference is switching the order of 'TripType' and 'VisitNumber'. 

Please keep me updated if and when this bug would be fixed. Thanks!
