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I have a shapefile with polygons and now, not sure why, most of the polygons became duplicated and overlaid, i.e. when i click to selected one, i get two selected... How do i identify those polygons and remove them?

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I don't know how to do it with Qgis itself, but you would be able to do it with GRASS.

See accepted reply to that post. Or have a look on the v.clean doc if you already are familiar to GRASS. See tool=rmdac, rmdup options.

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See MMQGIS Plugin with new tool (v2012.01.11) - Delete Duplicate Geometries

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If you only want to identify and safely remove them you should try kosmo or openjump with the QA extension. Or you can try a more automated way with the grass modules from qgis

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For QGIS 1.9.0 master (2.0) available plugin "Topology Checker". Links: Topology Checker plugin on YouTube, Topology Checker plugin on GitHub. There is included 'must not overlap' and 'must not have duplicates' topology rules.

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This plugin looks good for identifying problems - but how does it help the OP to remove the problems? – Simbamangu Oct 31 '13 at 8:02

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