I don't see why you should worry about someone 
who make a copy, if you have the original, that 
is all it matters, if someone would bother to 
copy or pirate a piece of art, perhaps you should 
be glad you own a piece of art someone would want 
it bad enough to make a copy.

decent artist creat original arts, not copying or 
pirate another image, there are plenty of awesome 
artist in Japan, not many would even bother to 
waste time and copy an image, maybe the character 
design, but definitely not copying an image, 
especially cels.

no matter how good someone claim they are at 
copying, it is completely impossible to make a 
fan cel that match the original 100%, even if 
they have the original douga, the best 
professional animators can't do it, they can make 
one that looks very similar or almost identical 
to the original, but there is will some tiny 
differences, there are highlight lines in front 
of the cel that needs to be hand inked, usually 
around character's hair, so the proportion, 
length will always be slightly different, if 
someone they to reproduce an image base on 
screencap, that would be even tougher, the 
proportion and color will be completely off.

one best example is the hand painted official 
reproduction cels, no two hand painted 
reproduction cel will match 100%, if you compare 
any hand painted reproduction cels side by side, 
you will find many minor differences.

Often the professional Japanese animators who 
painted the hanken cels, would make several copy 
of the originals for friends or to keep for 
themselves, this is reason often there would be a 
few fakes in the market, however it is extremely 
easy to tell when you compare the cel to the 
image where the cels is used.