But most copies are produced to deceive. Over the centuries the moment of danger usually came 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 too. Today we find of many paintings several versions in the collections of the world.
Very different works 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. 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 it with rather clumsily produced works and it still worked, tells a lot.
Details you will find on James Cahills webpage, for instance here.
Especially annoying is the case of changing a very good panting into a forgery by attaching later a different painters name to it. Often the attached name could not have done the quality of the original work attributed to him. And later the original artist might even turn into the more valuable artist.....
Scrolls are cut so that a new signature finds a place after the old is removed. If the scroll is long enough such a cut painting could create a number of forgeries.
It should be enough. the number of possibilities how a scoundrel may produce forgeries to his advantage is far too big to discuss them here all. The motivation behind all is never honest. The cheating should create value, in gold or political influence.
The collector, the connoisseur, now they should not think it is in any way possible to avoid this problem. Who is buying paintings, is sooner or later spending his money to buy a forgery. And if he buys in an expensive market, it only means he is paying more for it.
So the best approach would be, to buy at least a good painting!