forgeries       p.4

But most copies are produced to deceive. Over the centuries the moment of danger usually had arrived than the painting had to be mounted new. Now after the work is done a copy, and if the owner is stupid enough, he might even get back the copy mounted. Today also reproductions could replace the original as base for a copy and modern techniques of reproduction are used extensively.
This is a business and so we should expect a professional attitude of this industry, not only today, also long ago.

Unfortunately also big collectors, including the imperial collection, increased the pool of forgeries by producing copies after their fine original works to hand them out as presents and rewards. In the process the more professional painters who produced them exchanged what was possible to their own advantage. Today we find of many paintings several versions in the collections of the world.

Very different forgeries are produced by ghost painters.
They take a painter, study his brushwork and create new paintings imitating the individual brush of the master and sign them with his name. The unique work has no original able to denounce it. Once trained in a hand, paintings could be produced easier than sold.
This is even easier done in the case we dont know the individual brushwork of the painter forged, because no trustworthy works have survived.
The 20th century painter Zhang Daqian did this type of forgery for and together with a scrupulous and careless market including the most famous dealers and biggest collections in London and New York. That he did this with rather clumsily produced works of a mediocre quality and it still worked, tells a lot about the state of market and the academic world connected with it and the big public collections.
Details you will find again on James Cahills webpage,  for instance here.