Well it seems that the previous unresolved threat on this subject has been locked so hopefully I can place an update here.
Please refer to the original thread here: https://community.usa.canon.com/t5/Personal-Printers/My-printer-started-printing-quot-shadow-double-...
I have figured out what the problem is and it's 100% a fault with the large Black Pigment Ink part of the print head. It is mechanically different to the other print head jets because it has to deal with the pigment ink. I have the feeling that it jams up, causing this issue.
I came to this conclusion because I purchased a third MX926 which was used but in excellent condition. It was printing perfectly for the first six months and then, as I'd guessed, it started to print double. As it had just begun to do that, I noted that the large black ink cartridge was about 80% depleted, so I immediately removed it and put a new black cartridge in and re-primed the head again using a deep clean.
Surprisingly, the printer started to immediately print normally again.
I now print something with the large black head (pure black bold text) every second day and replace the black cartridge when it gets to about 70% depleted and so far, after another six months, it has continued to work perfectly.
I think I intervened just at the right time and I feel that as the ink thickens, it must somehow jam up the respective ink head jets. By supplying it with fresh and less viscous ink, it obviously restored operation. I feel that once the head is blocked, there's no restoring it. I've tried flushing the faulty heads using every known technique, including disassembly and pressurised cleaning solutions, all to no avail.
It's 100% definitely the black pigment ink jets in the print head that is the issue and I feel that as there is no fix, Canon don't want to acknowledge it.
Hope that helps.