Fix bug in flat data for completion_rate module

My commit message doesn't make a lot of sense, but it's late and I've been doing this for a while. I can take another look at it if you think it needs it.

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This `completion_rate` does not support the case where multiple pieces of data are used in the compilation of a single data point representing a given time period.

Additionally, this module would sometimes result in duplicated output in the table that accompanies a graph, for example on 'SORN':

    1 to 30 November 2013                    123%
    1 to 30 November 2013                    123%