Menu: Geometry->Edit->Uncollapse

The Uncollapse function lets you select lines, surfaces or volumes and duplicate all common lower entities.

Typically, if two surfaces share one line as an edge, after applying this function to both surfaces, that line and its shared points will be duplicated and every line will belong to a different surface.

This feature is interesting, for example, if you want to disconnect joined bodies or generate a non-conformal mesh with fewer elements than a conformal one.