Hello, I am wondering if I have two ships that have a crew slot can I have two C3PO crew. One on each ship?

Afraid not. Unique cards with dots can only be fielded once. IE, you can't have two Han Solos piloting, OR one Han piloting and one as crew. Now, your opponent can run a Han so you have Han vs. Han.

The dot before his name means "unique". You cannot have more than one card with a unique name in a list. That goes even for unique named cards of different types, e.g. you can't have R2-D2 (astromech) and R2-D2 (crew) in the same list.

You cannot have more than one card with a PARTICULAR unique name in a list.

You can have more than one unique card, as long as they have different names.

Also note that there are some unique upgrades that are not people. Lone wolf and squad leader, for example are also unique. As are many droids.