Reported at https://trac.osgeo.org/qgis/ticket/3558 by andskog:
When running the Points2One plugin with "Create output features based on input field" set, it errors if the point input file contains records with an input field value that doesn't occur more than once. Logical, as it "Can't make a plyline out of 1 points". But how about just skipping this point and keep on with the processing? It is awkward to go back to the dataset, try locating the point and deleting it, or exporting the other records to a new dataset for p2o-processing, if you have lots of these points occuring in the dataset. Ideally, one might want to keep the point file with all the data, if such once-occuring values are not data errors, and have a line dataset connecting the points that are supposed to be connected, created by this very useful plugin!
Fix commited in r47.