Duplicate nodes and adjasent areas

I see a lot of effort is being put into getting rid of duplicate nodes.

Excuse the noob question but what is so bad about duplicate nodes?

I have been looking at clearing some and I note quite a few are cause by 2 adjacent touching each other. How can duplicate nodes be avoided in this case?


Duplicate nodes are bad when two highways are supposed to intersect but don’t.
They should not be fixed when a highway crosses a boundary. When two boundaries intersect, it’s probably best to join them, but not to any intersecting roads.