problems dataflash logging of two columns with the same name

There seems to be an issue when logging two columns which have the same column title but come from different messages. For example MAG's MagZ column and MAG2's MagZ column. In particular at first two lines will be displayed when the checkboxes are checked but later if one check box is unchecked both lines will disappear.

Try this to reproduce the problem:
1. open up a .bin file produced by ArduCopter AC3.2 (or later) on a Pixhawk
2. on the right pane, click the checkboxes for MAG>MagZ and MAG2>MagZ
3. on the right pane, uncheck the MAG2>MagZ checkbox and both lines disappear

Reproduce the issue in Mission Planner with an ArduCopter AC3.2-or-later .bin file by selecting MAG>MagZ and MAG2>MagZ, then unchecking MAG2>MagZ. Trace the dataflash logging and checkbox handling for duplicate column titles; done means unchecking one column leaves the other plotted.

Written by the indexing model from the issue text.