reduce fails with duplicate column names

[dougbrn]

Bug report
reduce cannot handle duplicate column names in multiple nested dataframes, as it "flattens" all the input columns to a single dataframe. Fixing this should wait until #33 is resolved. Knowingly introduced by #34 .

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• I have described the situation in which the bug arose, including what code was executed, information about my environment, and any applicable data others will need to reproduce the problem.
• I have included available evidence of the unexpected behavior (including error messages, screenshots, and/or plots) as well as a descriprion of what I expected instead.
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[wilsonbb]

One potential solution here is to rename the columns when generating the merged frame to the form layer.column. Then before returning the result frame we rename all of the columns to drop the layer. prefix.

Renaming columns should in theory be cheap here

[dougbrn]

I don't think this is mission-critical for v0.1 so I'm opting to remove it from the milestone for now