Entering for Collections, when I own 1 game from the collection?

It is not against the rules. Some users ask that you not enter if you own a part of the pack.

It's up to whoever created the giveaway.

Creators discretion

It's usually worth asking the giveaway creator. Most won't mind if you own one or two parts of pack depending on its size. Obviously if you own 2 out of a 3 game pack, it may be worth leaving it to people who don't own so many parts for example. :)