Something of a subversion with Ping of Mega Tokyo, who is often thought to be an expy of Chii: they're both prototype consumer-product Robot Girls with similar appearance (short, long hair, weird ear devices, etc.) who fall in love with a male protagonist. Ping almost gets thrown in the trash; Hideki found Chii in the trash. The list goes on. But Chobits started serialization only a couple of months before Ping showed up, and the (remaining) author insists the similarity is just a coincidence.
An expy is necessarily proposital.