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This problem is similar to MDEV-10306.
This script:
SET optimizer_switch=_utf8'derived_merge=on';
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DROP TABLE IF EXISTS t1;
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CREATE TABLE t1 (t VARCHAR(10) CHARSET latin1);
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INSERT INTO t1 VALUES('abcdefghi');
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SELECT CONCAT(t2,'-',t2) c2 FROM (SELECT MAKE_SET(3,t,t) t2 FROM t1) sub;
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returns a wrong result:
+----------------------------------------+
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| c2 |
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+----------------------------------------+
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| abcdefghi,abcdefghiabcdefghi,abcdefghi |
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+----------------------------------------+
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Notice, there is no the expected dash character in the result.
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