In some dance communities, primarily modern urban contra ones but I've seen this in scottish and english dance too, it's rare for people to dance with each other for more than one dance. Sometimes they will, especially spouses. A few couples dance only with each other, which will occasionally elicit grumbles from people who wish they were dancing with one of them but is completely fine. It's not too common, though.

Often a two new people will arrive together and either want to or think that they're supposed to dance only with each other. Just from the perspective of learning how to contra dance, it would be better if they were to dance with many different partners -- people usually learn faster that way.

So callers will sometimes see this and announe from the stage that people aren't supposed to keep dancing with the same partner over and over. Not only does this rarely work, it is too heavy handed; without talking to the newcomers, how do you know why they keep dancing togethter? If they're dancing together because they want to, well then let them. If they're doing it because they're afraid (they think they have to; they're afraid to of disappointing a partner who knows what they're doing; they're afraid that if they get another partner then the person they came with won't have one) that's not so good.

The best solution I've seen is for two experienced dancers to walk over to the couple after a dance ends and each ask one of them individually if they want to dance. This lets them see that the person they came with will still have a partner and that switching partners is fine, while giving them the chance to say "no, we'd rather dance with each other" (which should be accepted by the experienced dancers).