There is such a think as a knockoff or imitation, which is:
- similar to, but not identical to, a brand name, or
- inspired by it
and this is legitimate to buy.
There is such a thing as a phony version of a brand product, a version being passed off as genuine, also sometimes referred to as a "knockoff".
To knowingly purchase such a phony thing makes one an accessory to the crime committed by the fraudulent manufacturer and marketer, and an accessory to the sin of theft that they are committing, by stealing the:
- design, and
- reputation of the product.
If you bought something fraudulent and did not know it, you may keep it.
I hope this helps.