Merging issue
I have two dataframes.

sharesdb:

```
                                       PARTICIPANTNAME   2015-06-28
```

0  THE HONGKONG AND SHANGHAI BANKING   765945421
1  THE HONGKONG AND SHANGHAI BANKING   398945696

tmpdb:

```
                                       PARTICIPANTNAME   2015-06-29
```

0  THE HONGKONG AND SHANGHAI BANKING   766433989
1  THE HONGKONG AND SHANGHAI BANKING   398817122

Then I run the following:

pd.merge(sharesdb, tmpdb, on='PARTICIPANTNAME', how='outer')

The result is:

```
                                       PARTICIPANTNAME   2015-06-28  2015-06-29
```

0  THE HONGKONG AND SHANGHAI BANKING   765945421   766433989
1  THE HONGKONG AND SHANGHAI BANKING   765945421   398817122
2  THE HONGKONG AND SHANGHAI BANKING   398945696   766433989
3  THE HONGKONG AND SHANGHAI BANKING   398945696   398817122

However, what I expected should be:

```
                                       PARTICIPANTNAME   2015-06-28  2015-06-29
```

0  THE HONGKONG AND SHANGHAI BANKING   765945421   766433989
1  THE HONGKONG AND SHANGHAI BANKING   398945696   398817122

Is it a bug?
