…For example, if there is a Goal which requires two or three of several, can two of the same card fulfill this goal?

A: No, you need the Keepers which are fulfilling the Goal to be unique in function, unless it is specified that they may or must be the same function, but with some some other specific difference (like some Goals from the Bridge Pack which specifically call for Keepers of the same name, but different eras).

Also see this relevant question about combining Trek Fluxxen: https://faq.looneylabs.com/question/2065