bugs - A unexpected behavior in RelationGraph

Bug introduced in 10.2 and fixed in 11.0.1

---

Firstly we get these idiom

web = Import["http://chengyu.t086.com/jielong/zuichang.html"];

idiom = StringCases[web, 
   RegularExpression[
    "(?<=\\s\\-\\>\\s)[一-龥]{4}|[一-龥]{4}(?=\\s\\-\\>\\s)"]];

Then he use the RelationGraph to draw graph.

g = RelationGraph[StringTake[#1, -1] == StringTake[#2, 1] &, 
  idiom[[;; 1664]]]

We can get

Actually the idiom have more element.

idiom // Length

1792

But If I want to get more long graph upto 1665

g = RelationGraph[StringTake[#1, -1] == StringTake[#2, 1] &, 
      idiom[[;; 1665]]]

We'll get a unexpected graph or a error information,But I cannot find any bad stuff surround 1655

Take[idiom, {1663, 1667}]

{依依不舍,舍己为人,轻而易举,举一反三,三生有幸}

Is it a bug of RelationGraph or have I missed something?

Answer

There is a duplicated vertex causing this issue:

idiom[[;; 1665]] // Length

1665

idiom[[;; 1665]] // Union // Length

1664

As a workaround, you could take Union over the vertex set:

g = RelationGraph[StringTake[#1, -1] == StringTake[#2, 1] &, 
   Union@idiom[[;; 1665]]];

GraphQ[g]

True

but the output shouldn't be that.

You should report this to Wolfram support.