Forbid duplicate edges in Graph
Created by: gkirgizov
Currently Graph is actually a multi-graph. It silently accepts nodes that have duplicate entries in node_from, but doesn't handle this case on deletion and updates of nodes/subtrees. This leads to far-reaching issues such as #580 (closed).
Because there's no use case for multi-graphs, the proposal is to process this case explicitly, forbidding graphs with duplicate edges. Discussed and decided on it with @Dreamlone.