Assume you have two snapshots for /dir: s1, s2, when you do "hdfs dfs
-renameSnapshot /dir s2 s1" which means you want to rename s2 to s1. It will cause s2 been dropped silently and s1 still keep the previous one. This is very confusing and easily cause wrong operation.
The right behavior should be providing a warning to remind user that the target name is already existed and do nothing. If user do want to overlap s1, he should add some option (i.e. "-f") or delete s1 first.