Bug introduced in 10.0 or earlier and fixed in 12.1
M10.0.2 and M10.3 return a wrong result without errors. M11.0 and M12.0 issue a flood of messages and sometimes crash.

I am using Mathematica 12.0, and the code

EdgeAdd[Graph@{UndirectedEdge[1, 2], UndirectedEdge[1, 2]}, UndirectedEdge[1, 1]]

results in some weird error ("Take: Cannot take positions 1 through 4 in ..."). Adding edges to simple graphs seems to be OK, so

EdgeAdd[Graph@{UndirectedEdge[1, 2]}, UndirectedEdge[1, 1]]

works fine. Also there seems to be no issue with having multigraphs with self-loops

Graph@{UndirectedEdge[1, 2], UndirectedEdge[1, 2], UndirectedEdge[1, 1]}

so the resulting graph itself has no problems. Any idea what is going on here?



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