Support duplicate columns in csv network builder

[jjnesbitt]

There's a reasonable use case where the user has one node table and one edge table. Each edge table's from and to columns could both point to the same column in that one node table. As it currently stands, this at least isn't supported in the GUI. It might not be an issue in the backend, but the GUI currently prevents this action.

This should be updated to allow for this.

[waxlamp]

Agreed. This is always a valid use case AFAICT.