Ignore overlapping edges/nodes?

If the user of the library attempts to add an edge between two nodes where an edge already exists, should we ignore it? Similarly, if someone adds a node at the same coordinates as an existing node, should we ignore it? On the one hand, if we ignore, we don't end up with unnecessary geometries. On the other hand, we are not performing the task the user requested and it's their fault for adding overlapping geometries.