Improve output when trying to create features that already exist - possible enhancement

### Improve output when trying to create features that already exist
This is not a bug, but could be a potential enhancement.

#### Feature Request Description
If dfs attempts to create a feature with a name that is already used on an existing column, a warning similar to the one below is generated:
```
2019-12-03 12:30:34,559 featuretools - WARNING Attempting to add feature which is already present. This is likely a bug.
```
This could happen in a couple situations:
1. The data contains a column that has the same name as the name being auto-generated by featuretools.
2. The user attempts to run dfs on an existing feature matrix that still contains the original data columns.

These situations are illustrated in the attached notebook.

[duplicate_feature_names.ipynb.zip](https://github.com/FeatureLabs/featuretools/files/3918115/duplicate_feature_names.ipynb.zip)

It might be good to have better handling of these situations, or improved error messages. Maybe dfs could have a parameter that controls the behavior if existing features are found (overwrite existing with new or leave existing unchanged).