When working with Graphics APIs like OpenGL its common to use an index face set model. That is, multiple faces share adjacent / overlapping vertices by using a list of indices.
Most game engines like Unity come with an integrated lightmapper that provides automatic UV unwrapping (UVs for lightmaps are usually different from those for the albedo texture).
However, automatic UV unwrapping (or UV unwrapping in general) means almost certainly seams which means a single vertex (along the seam) must have multiple UV coordinates (one for each face) if the faces are laid out in different charts in the UV atlas.
How is this dealt with in like Blender or Unity? Does the process of UV unwrapping add more vertices and modify the model? At least I'm not aware that UV unwrapping in Unity for lightmap generation changes my model.