See the Pick Up Points (PUPs) arrows, pair one with double transfer (right stamp) and one without double transfer (left stamp). The PUPs will point to the double transfer.

The first issue revenue series of 1862, you can find numerous double transfers.

I found this one on Ebay, the seller did not realize this was a double transfer.

I don't buy double transfers as such, I discover them in dealer stocks that are unrecoginized as such.

You can also find unrecognized "foreign design transfers", "cracked plates", "tool marks", "shifted transfers", more etc.