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... If either list-1 or list-2 has duplicate entries within it, the redundant entries might or might not appear in the result ...

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Yes, you're right. The fact that they leave the behavior of union in this case undefined in the standard is surprising to me. The fact that SBCL and GNU CLISP implement it the way they do is just bizarre to me.

It was probably left in for better performance. Glad I could help.

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