Yep, I have had the same miserable time of it too with a 3 year old occasionaly home used Cannon PIXMA iP8750. After trying every maintenance method i.e. deep cleaning, auto head alignment, manual head alignment and ink cartridge replacement, the black ink head still printed "shadow/double" letters, so nothing worked as has been reported over a range of Cannon printers that use this model printer head. (Genuine Canon Print Head QY6-0083-000 £70-£90) Just to use up my ink and give me time to research into a new printer I came up with this idea for word documents. And that is to set the default colour as ‘Dark Blue’ (or any colour you like really) For existing word documents, highlight all the text and change the font colour to Dark Blue. As it seems this Canon Printer fault does not affect the colour heads the font remains clear and crisp. I hope this helps those in desperation!
All the best
gustav2