I've been asked a couple of times to make "copies" of hats or clothes people already have.

Sometimes this is possible, and sometimes it isn't.   Almost everything shop bought is machine knitted, which is MUCH quicker, and can produce effects that simply aren't feasible by hand. It depends on a few things, including the bulkiness of the yarn and the pattern. 

Light-weight sweaters are often made from incredibly fine yarn that would take MONTHS to reproduce. 

Lots of things that look like patterns are in fact printed on, and can't be reproduced by hand. 

Aran-style knits in shops are often sewn together from several pieces, not knitted as a whole. 

Fair Isle style jumpers and accessories are hugely popular at the moment, and often more complicated (having been made on a machine) that they look, which can made identical reproduction a bit tricky.

 But show it to me and I'll see what I can do! It might not be exactly the same, but it might not be too far off.  One warning though - a hand knitted reproduction will almost always cost more than the original.