If you merge one than one time in sequence the same sources (merged data not changed between runs) you'll get the same results. Period
I had to commit to trunk, and merge it to branch many times today, becuase lots of bugs I made...
If you "commit-merge-commit-merge...-commit merge" you have changed at least one source, merge-result will differ in this case. What's so surprising to you?
Anyway
merge result is not reliable!
is not good description of problem, even not description of problem at all. Stop hysterics and show as much details, as you can. Something like this (I'll try just imagine your case)
Sample Bug Report BEGIN
When I try to merge URL/trunk/filename@REVA
with URL/branches/somebranch/filename@REVB
I always get different merge results, see
SAMPLEMERGED1
SAMPLEMERGED2
SAMPLEMERGED3
even with the same diff all times
svn diff URL/trunk/filename@REVA URL/branches/somebranch/filename@REVB
... diff output here ...
History of branch file is
svn log --stop-on-copy URL/branches/somebranch/filename@REVB
... log output here ...
I have TortoiseSVN v. NNN and repository on SVN-server v. NNN
Sample Bug Report END