Github user dkuppitz commented on a diff in the pull request:
https://github.com/apache/tinkerpop/pull/531#discussion_r95639012
--- Diff: docs/src/recipes/duplicate-edge.asciidoc ---
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+////
+Licensed to the Apache Software Foundation (ASF) under one or more
+contributor license agreements. See the NOTICE file distributed with
+this work for additional information regarding copyright ownership.
+The ASF licenses this file to You under the Apache License, Version 2.0
+(the "License"); you may not use this file except in compliance with
+the License. You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
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+See the License for the specific language governing permissions and
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+////
+[[duplicate-edge]]
+Duplicate Edge Detection
+------------------------
+
+Whether part of a graph maintenance process or for some other analysis
need, it is sometimes necessary to detect
+if there is more than one edge between two vertices. The following
examples will assume that an edge with the same
+label and direction will be considered "duplicate".
+
+The "modern" graph does not have any duplicate edges that fit that
definition, so the following example adds one
+that is duplicative of the "created" edge between vertex "1" and "3".
+
+[gremlin-groovy,modern]
+----
+g.V(1).as("a").V(3).addE("created").from("a").iterate()
+g.V(1).outE("created")
+----
+
+One way to find the duplicate edges would be to do something like this:
+
+[gremlin-groovy,existing]
+----
+g.V().outE().
+ project("a","b"). <1>
+ by().by(inV().path().by().by(label)).
+ group(). <2>
+ by(select("b")).
+ by(select("a").fold()).
+ unfold(). <3>
+ select(values). <4>
+ where(count(local).is(gt(1)))
+----
+
+<1> The "a" and "b" from the `project` contain the edge and the path
respectively. The path consists of a the outgoing
+vertex, an edge, and the incoming vertex. The use of `by().by(label))`
converts the edge to its label (recall that `by`
+are applied in round-robin fashion), so the path will look something like:
`[v[1],created,v[3]]`.
+<2> Group by the path from "b" and construct a list of edges from "a". Any
value in this `Map` that has a list of edges
+greater than one means that there is more than one edge for that edge
label between those two vertices (i.e. the `Map`
+key).
+<3> Unroll the key-value pairs in the `Map` of paths-edges.
+<4> Only the values from the `Map` are needed and as mentioned earlier,
those lists with more than one edge would
+containa duplicate.
+
+This method find the duplicates, but does require more memory than other
approaches. A slightly more complex approach
+that uses less memory might look like this:
+
+[gremlin-groovy,existing]
+----
+g.V().as("ov").
--- End diff --
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