Don't append the tombstone to the update chain if it is already there

format.sh: /home/chenhaoqu/work/wiredtiger/test/format/RUNDIR.5 log:

    t: process 24909 running

    [1588032020:605942][24909:0x7ff087fff700], t, file:wt, WT_SESSION.commit_transaction: __wt_hs_insert_updates, 714: prev_upd->type == WT_UPDATE_STANDARD

    [1588032020:605974][24909:0x7ff087fff700], t, file:wt, WT_SESSION.commit_transaction: __wt_abort, 28: aborting WiredTiger library