I am currently working in Qgis (2.18.11) and I would like to view mi map window duplicated so I can represent a thematic map with different values in each window and compare both.
I usually do this in MapInfo (Map > Clone view). See the example:
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Not exactly the same, but you can duplicate the layer (right click > 'Duplicate') which adds the layer again (as opposed to making a copy) which you can then style in a different way. If you put the layer and its duplicate into a group and then make the group mutually exclusive ( needs a reasonably recent version of QGIS) you can then toggle between the styling options.