Ticket for the trouble described at

Error:
<verbatim>
.... [many updates]
RENAME a.cmd -> a2.cmd
fossil: SQLITE_CONSTRAINT: abort at 34 in [UPDATE vfile SET
pathname='a2.cmd', origname=coalesce(origname,pathname) WHERE
id=1793175 AND vid=14998]: columns pathname, vid are not unique
fossil: columns pathname, vid are not unique
.... [many undo]
fossil: SQLITE_BUSY: statement aborts at 2: [ROLLBACK] cannot rollback
transaction - SQL statements in progress
</verbatim>

Running with "merge --debug trunk":
<verbatim>
P->M at 15956> c0224914b6: 4198[a2.cmd] -> 0[]
P->M at 15957> 9d69a325f1: 2405[a.cmd] -> 0[]
P->M at 15957> 9d69a325f1: 2405[a.cmd] ->
4198[a2.cmd]
P->M summary 2405[a.cmd] -> 4198[a2.cmd]
186: ridv=151  ridp=151  ridm=15973 chnged=0 isexe=0
    fn  = [a2.cmd]
    fnp = [a2.cmd]
    fnm = [a2.cmd]
187: ridv=14576 ridp=14576 ridm=15973 chnged=0 isexe=0
    fn  = [a.cmd]
    fnp = [a.cmd]
    fnm = [a2.cmd]
</verbatim>