statistics: asyncMergeGlobalStats wrongly merges the same cmsketch twice

[L-maple]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

func TestGlobalStatsDataV2(t *testing.T) {
	store, _ := testkit.CreateMockStoreAndDomain(t)
	tk := testkit.NewTestKit(t, store)
	tk.MustExec("use test")
	tk.MustExec("drop table if exists t")
	tk.MustExec(`
		create table t (a int) partition by range (a) (
			partition p0 values less than (10),
			partition p1 values less than (20)
		)`)
	tk.MustExec("set @@tidb_analyze_version=1")
	tk.MustExec("set @@tidb_partition_prune_mode='dynamic'")
	tk.MustExec("insert into t values (1), (2), (3), (4), (5), (6), (6), (null), (11), (12), (13), (14), (15), (16), (17), (18), (19), (19)")
	tk.MustExec("analyze table t")
	tk.MustQuery("explain select * from t where a = 1").Check(testkit.Rows(""))
}

2. What did you expect to see? (Required)

The estimated rows should be one.

TableReader_7 1.00 root partition:p0 data:Selection_6
└─Selection_6 1.00 cop[tikv]  eq(test.t.a, 1)
 └─TableFullScan_5 18.00 cop[tikv] table:t keep order:false

3. What did you see instead (Required)

The estimated rows should not be two.

TableReader_7 2.00 root partition:p0 data:Selection_6
└─Selection_6 2.00 cop[tikv]  eq(test.t.a, 1)
  └─TableFullScan_5 18.00 cop[tikv] table:t keep order:false

The root cause of this problem is, the cmsketch is merged twice! L391~393 should be removed.

4. What is your TiDB version? (Required)

mysql> select tidb_version();
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version() |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v9.0.0-alpha-158-gba79f5078d-dirty
Edition: Community
Git Commit Hash: ba79f50
Git Branch: master
UTC Build Time: 2025-01-24 08:44:02
GoVersion: go1.23.5
Race Enabled: false
Check Table Before Drop: false
Store: unistore |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+