Duplicate clefs when importing MusicXML created by NoteFlight
Create a basic section of music in NoteFlight - a single bar will do. Export to MusicXML. Import into MuseScore. Note that what should be the bass clef has a treble clef sign initially and then switches. It appears that on both there is an initial treble clef before the imported data begins. I've attached screenshots of the original in NoteFlight and the difference between importing into MuseScore vs Sibelius First.
There is a sort of workaround - the second clefs can be deleted and a bass clef then dragged over the treble clef in the lower part of the stave. This does still leave a space between the clefs and key signatures that shouldn't be there. I haven't yet tested to see if saving and reopening the file fixes the problem.