13dev failed assert: comparetup_index_btree(): ItemPointer values should never be equal

An internal server aborted last night while running a maintenance script.  I
reproduced this easily running the crashing command in a loop, and verified
this is a live issue on REL_13_STABLE (dc6f2fb43).
REINDEX INDEX pg_class_tblspc_relfilenode_index

It looks like this crashed once before, and I didn't notice until now:
Fri Jun 26 22:30:29 CDT 2020: pg_shdescription: pg_toast.pg_toast_2396_index(reindex toast)...
psql: error: could not connect to server: server closed the connection unexpectedly

#3  0x0000000000afde98 in comparetup_index_btree (a=0x201fa58, b=0x201fa10, state=0x20147b0) at tuplesort.c:4251
4251            Assert(false);
(gdb) l
4246                    if (pos1 != pos2)
4247                            return (pos1 < pos2) ? -1 : 1;
4248            }
4249
4250            /* ItemPointer values should never be equal */
4251            Assert(false);
4252
4253            return 0;
4254    }
4255

#3  0x0000000000afde98 in comparetup_index_btree (a=0x201fa58, b=0x201fa10, state=0x20147b0) at tuplesort.c:4251
        sortKey = 0x2014d60
        tuple1 = 0x20189d8
        tuple2 = 0x2018898
        keysz = 2
        tupDes = 0x7f48996b3790
        equal_hasnull = false
        nkey = 3
        compare = 0
        datum1 = 67999603
        datum2 = 67999603
        isnull1 = false
        isnull2 = false
        __func__ = "comparetup_index_btree"

(gdb) p *tuple1
$2 = {t_tid = {ip_blkid = {bi_hi = 0, bi_lo = 0}, ip_posid = 43}, t_info = 16}
(gdb) p *tuple2
$3 = {t_tid = {ip_blkid = {bi_hi = 0, bi_lo = 0}, ip_posid = 43}, t_info = 16}

(gdb) p *sortKey
$5 = {ssup_cxt = 0x40, ssup_collation = 0, ssup_reverse = false, ssup_nulls_first = false, ssup_attno = 0, ssup_extra =
0x0,comparator = 0x7f7f7f7f7f7f7f7f, abbreviate = 127, abbrev_converter = 0x7f7f7f7f7f7f7f7f, 
 
  abbrev_abort = 0x7f7f7f7f7f7f7f7f, abbrev_full_comparator = 0x7f7f7f7f7f7f7f7f}

(gdb) p *tupDes
$6 = {natts = 2, tdtypeid = 0, tdtypmod = -1, tdrefcount = 1, constr = 0x0, attrs = 0x7f48996b37a8}

Core was generated by `postgres: postgres sentinel [local] REINDEX            '.

(gdb) bt
#0  0x00007f489853d1f7 in raise () from /lib64/libc.so.6
#1  0x00007f489853e8e8 in abort () from /lib64/libc.so.6
#2  0x0000000000aafff7 in ExceptionalCondition (conditionName=0xccd0dc "false", errorType=0xccc327 "FailedAssertion",
fileName=0xccc2fd"tuplesort.c", lineNumber=4251) at assert.c:67
 
#3  0x0000000000afde98 in comparetup_index_btree (a=0x201fa58, b=0x201fa10, state=0x20147b0) at tuplesort.c:4251
#4  0x0000000000af1d5e in qsort_tuple (a=0x201fa10, n=18, cmp_tuple=0xafcf21 <comparetup_index_btree>, state=0x20147b0)
atqsort_tuple.c:140
 
#5  0x0000000000af2104 in qsort_tuple (a=0x201f710, n=50, cmp_tuple=0xafcf21 <comparetup_index_btree>, state=0x20147b0)
atqsort_tuple.c:191
 
#6  0x0000000000af2104 in qsort_tuple (a=0x201cc38, n=544, cmp_tuple=0xafcf21 <comparetup_index_btree>,
state=0x20147b0)at qsort_tuple.c:191
 
#7  0x0000000000af8056 in tuplesort_sort_memtuples (state=0x20147b0) at tuplesort.c:3490
#8  0x0000000000af51a9 in tuplesort_performsort (state=0x20147b0) at tuplesort.c:1985
#9  0x0000000000529418 in _bt_leafbuild (btspool=0x1f784e0, btspool2=0x0) at nbtsort.c:553
#10 0x0000000000528f9c in btbuild (heap=0x1fb5758, index=0x7f48996b3460, indexInfo=0x1f77a48) at nbtsort.c:333
#11 0x00000000005adcb3 in index_build (heapRelation=0x1fb5758, indexRelation=0x7f48996b3460, indexInfo=0x1f77a48,
isreindex=true,parallel=true) at index.c:2903
 
#12 0x00000000005aec6b in reindex_index (indexId=3455, skip_constraint_checks=false, persistence=112 'p', options=2) at
index.c:3539
#13 0x0000000000692583 in ReindexIndex (indexRelation=0x1f54840, options=0, concurrent=false) at indexcmds.c:2466
#14 0x0000000000932e36 in standard_ProcessUtility (pstmt=0x1f54960, queryString=0x1f53d90 "REINDEX INDEX
pg_class_tblspc_relfilenode_index",context=PROCESS_UTILITY_TOPLEVEL, params=0x0, queryEnv=0x0, dest=0x1f54c40,
 
    qc=0x7ffde023bf80) at utility.c:929
#15 0x000000000093241f in ProcessUtility (pstmt=0x1f54960, queryString=0x1f53d90 "REINDEX INDEX
pg_class_tblspc_relfilenode_index",context=PROCESS_UTILITY_TOPLEVEL, params=0x0, queryEnv=0x0, dest=0x1f54c40,
qc=0x7ffde023bf80)
    at utility.c:524
#16 0x0000000000931288 in PortalRunUtility (portal=0x1fb5ac0, pstmt=0x1f54960, isTopLevel=true, setHoldSnapshot=false,
dest=0x1f54c40,qc=0x7ffde023bf80) at pquery.c:1157
 
#17 0x00000000009314a7 in PortalRunMulti (portal=0x1fb5ac0, isTopLevel=true, setHoldSnapshot=false, dest=0x1f54c40,
altdest=0x1f54c40,qc=0x7ffde023bf80) at pquery.c:1303
 
#18 0x00000000009309bc in PortalRun (portal=0x1fb5ac0, count=9223372036854775807, isTopLevel=true, run_once=true,
dest=0x1f54c40,altdest=0x1f54c40, qc=0x7ffde023bf80) at pquery.c:779
 
#19 0x000000000092ab5b in exec_simple_query (query_string=0x1f53d90 "REINDEX INDEX pg_class_tblspc_relfilenode_index")
atpostgres.c:1239
 
#20 0x000000000092eb82 in PostgresMain (argc=1, argv=0x1f7db80, dbname=0x1f509d8 "sentinel", username=0x1f7daa0
"pryzbyj")at postgres.c:4315
 
#21 0x000000000087f098 in BackendRun (port=0x1f75a80) at postmaster.c:4523
#22 0x000000000087e888 in BackendStartup (port=0x1f75a80) at postmaster.c:4215
#23 0x000000000087ae95 in ServerLoop () at postmaster.c:1727
#24 0x000000000087a76c in PostmasterMain (argc=5, argv=0x1f4e8e0) at postmaster.c:1400
#25 0x00000000007823f3 in main (argc=5, argv=0x1f4e8e0) at main.c:210

This appears to be an issue with d2d8a229b (Incremental Sort), so I will add
at: https://wiki.postgresql.org/wiki/PostgreSQL_13_Open_Items