iTunes Match will recognise previously matched and uploaded songs so should not create duplicates.
I have not had to readd complete library but have accidentally deleted a few hundred tracks. When I readded the match process was very quick. Previously uploaded tracks did not go through normal upload process as match recognised them
One oddity was that some previously matched track were now uploaded.
No duplicate entries were made.
Ok that didnt work out quite as planned. On the positive iTunes match recgnoised any duplicates and highlighted them so they were easliy deleted. On the negative any name changes or album consolidation that I had previsouly made was removed, my libary looks like a bomb blew up in it. Alot of organising to do