I think this is a coincidence.

We met completely by coincidence.

Canadian Thanksgiving Day and Columbus Day in the USA coincide, so Esperanto speakers from both countries take this opportunity to have an international meeting.

It must be a coincidence.

That can't be a coincidence.

It's a fortunate coincidence.

Are you sure it is a coincidence?

That's quite a coincidence.

It's more than a coincidence.

It was just a coincidence.