it's a small world

One encounters the same people, events or situations in unexpected places. For example, I never thought I'd run into Samantha at a ball game—it's a small world. [c. 1900]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.