More descriptive error when columns are duplicated #11
Description
On https://github.com/cjohnhanson/schematic/blob/master/schematic/base.py#L204, it would be helpful if the error said which columns are appearing more than once
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On https://github.com/cjohnhanson/schematic/blob/master/schematic/base.py#L204, it would be helpful if the error said which columns are appearing more than once